This is v5 of Syscall User Dispatch. It has some big changes in comparison to v4.
First of all, it allows the vdso trampoline code for architectures that support it. This is exposed through an arch hook. It also addresses the concern about what happens when a bad selector is provided, instead of SIGSEGV, we fail with SIGSYS, which is more debug-able.
Another major change is that it is now based on top of Gleixner's common syscall entry work, and is supposed to only be used by that code. Therefore, the entry symbol is not exported outside of kernel/entry/ code.
The biggest change in this version is the attempt to avoid using one of the final TIF flags on x86 32 bit, without increasing the size of that variable to 64 bit. My expectation is that, with this work, plus the removal of TIF_IA32, TIF_X32 and TIF_FORCE_TF, we might be able to avoid changing this field to 64 bits at all. Instead, this follows the suggestion by Andy to have a generic TIF flag for SECCOMP and this mechanism, and use another field to decide which one is enabled. The code for this is not complex, so it seems like a viable approach.
Finally, this version adds some documentation to the feature.
Kees, I dropped your reviewed-by on patch 5, given the amount of changes.
Thanks,
Previous submissions are archived at: RFC/v1: https://lkml.org/lkml/2020/7/8/96 v2: https://lkml.org/lkml/2020/7/9/17 v3: https://lkml.org/lkml/2020/7/12/4 v4: https://lwn.net/ml/linux-kernel/20200712044516.2347844-1-krisman@collabora.c...
Gabriel Krisman Bertazi (9): kernel: Support TIF_SYSCALL_INTERCEPT flag kernel: entry: Support TIF_SYSCAL_INTERCEPT on common entry code x86: vdso: Expose sigreturn address on vdso to the kernel signal: Expose SYS_USER_DISPATCH si_code type kernel: Implement selective syscall userspace redirection kernel: entry: Support Syscall User Dispatch for common syscall entry x86: Enable Syscall User Dispatch selftests: Add kselftest for syscall user dispatch doc: Document Syscall User Dispatch
.../admin-guide/syscall-user-dispatch.rst | 87 ++++++ arch/Kconfig | 21 ++ arch/x86/Kconfig | 1 + arch/x86/entry/vdso/vdso2c.c | 2 + arch/x86/entry/vdso/vdso32/sigreturn.S | 2 + arch/x86/entry/vdso/vma.c | 15 + arch/x86/include/asm/elf.h | 1 + arch/x86/include/asm/thread_info.h | 4 +- arch/x86/include/asm/vdso.h | 2 + arch/x86/kernel/signal_compat.c | 2 +- fs/exec.c | 8 + include/linux/entry-common.h | 6 +- include/linux/sched.h | 8 +- include/linux/seccomp.h | 20 +- include/linux/syscall_intercept.h | 71 +++++ include/linux/syscall_user_dispatch.h | 29 ++ include/uapi/asm-generic/siginfo.h | 3 +- include/uapi/linux/prctl.h | 5 + kernel/entry/Makefile | 1 + kernel/entry/common.c | 32 +- kernel/entry/common.h | 15 + kernel/entry/syscall_user_dispatch.c | 101 ++++++ kernel/fork.c | 10 +- kernel/seccomp.c | 7 +- kernel/sys.c | 5 + tools/testing/selftests/Makefile | 1 + .../syscall_user_dispatch/.gitignore | 2 + .../selftests/syscall_user_dispatch/Makefile | 9 + .../selftests/syscall_user_dispatch/config | 1 + .../syscall_user_dispatch.c | 292 ++++++++++++++++++ 30 files changed, 744 insertions(+), 19 deletions(-) create mode 100644 Documentation/admin-guide/syscall-user-dispatch.rst create mode 100644 include/linux/syscall_intercept.h create mode 100644 include/linux/syscall_user_dispatch.h create mode 100644 kernel/entry/common.h create mode 100644 kernel/entry/syscall_user_dispatch.c create mode 100644 tools/testing/selftests/syscall_user_dispatch/.gitignore create mode 100644 tools/testing/selftests/syscall_user_dispatch/Makefile create mode 100644 tools/testing/selftests/syscall_user_dispatch/config create mode 100644 tools/testing/selftests/syscall_user_dispatch/syscall_user_dispatch.c
Convert TIF_SECCOMP into a generic TI flag for any syscall interception work being done by the kernel. The actual type of work is exposed by a new flag field outside of thread_info. This ensures that the syscall_intercept field is only accessed if struct seccomp has to be accessed already, such that it doesn't incur in a much higher cost to the seccomp path.
In order to avoid modifying every architecture at once, this patch has a transition mechanism, such that architectures that define TIF_SECCOMP continue to work by ignoring the syscall_intercept flag, as long as they don't support other syscall interception mechanisms like the future syscall user dispatch. When migrating TIF_SECCOMP to TIF_SYSCALL_INTERCEPT, they should adopt the semantics of checking the syscall_intercept flag, like it is done in the common entry syscall code, or even better, migrate to the common syscall entry code.
This was tested by running the selftests for seccomp. No regressions were observed, even though I had 4 tests consistently failing (with and without this patch).
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- include/linux/sched.h | 6 ++- include/linux/seccomp.h | 20 ++++++++- include/linux/syscall_intercept.h | 70 +++++++++++++++++++++++++++++++ kernel/fork.c | 10 ++++- kernel/seccomp.c | 7 ++-- 5 files changed, 106 insertions(+), 7 deletions(-) create mode 100644 include/linux/syscall_intercept.h
diff --git a/include/linux/sched.h b/include/linux/sched.h index 52bcc9f48e17..23b3e155ccab 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -954,7 +954,11 @@ struct task_struct { kuid_t loginuid; unsigned int sessionid; #endif - struct seccomp seccomp; + + struct { + unsigned int syscall_intercept; + struct seccomp seccomp; + };
/* Thread group tracking: */ u64 parent_exec_id; diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h index 02aef2844c38..027dc462cea9 100644 --- a/include/linux/seccomp.h +++ b/include/linux/seccomp.h @@ -20,6 +20,24 @@ #include <linux/atomic.h> #include <asm/seccomp.h>
+/* + * Some transitional defines to avoid migrating every architecture code + * at once. + */ + +#if defined(TIF_SECCOMP) && defined(TIF_SYSCALL_INTERCEPT) +# error "TIF_SYSCALL_INTERCEPT and TIF_SECCOMP can't be defined at the same time" +#endif + +/* + * If the arch has not transitioned to TIF_SYSCALL_INTERCEPT, this let + * seccomp work with these architectures, as long as no other syscall + * intercept features are meant to be supported. + */ +#ifdef TIF_SECCOMP +# define TIF_SYSCALL_INTERCEPT TIF_SECCOMP +#endif + struct seccomp_filter; /** * struct seccomp - the state of a seccomp'ed process @@ -42,7 +60,7 @@ struct seccomp { extern int __secure_computing(const struct seccomp_data *sd); static inline int secure_computing(void) { - if (unlikely(test_thread_flag(TIF_SECCOMP))) + if (unlikely(test_thread_flag(TIF_SYSCALL_INTERCEPT))) return __secure_computing(NULL); return 0; } diff --git a/include/linux/syscall_intercept.h b/include/linux/syscall_intercept.h new file mode 100644 index 000000000000..725d157699da --- /dev/null +++ b/include/linux/syscall_intercept.h @@ -0,0 +1,70 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright (C) 2020 Collabora Ltd. + */ +#ifndef _SYSCALL_INTERCEPT_H +#define _SYSCALL_INTERCEPT_H + +#include <linux/sched.h> +#include <linux/sched/signal.h> +#include <linux/thread_info.h> + +#define SYSINT_SECCOMP 0x1 + +#ifdef TIF_SYSCALL_INTERCEPT + +/* seccomp (at least) can modify TIF_SYSCALL_INTERCEPT from a different + * thread, which means it can race with itself or with + * syscall_user_dispatch. Therefore, TIF_SYSCALL_INTERCEPT and + * syscall_intercept are synchronized by tsk->sighand->siglock. + */ + +static inline void __set_tsk_syscall_intercept(struct task_struct *tsk, + unsigned int type) +{ + tsk->syscall_intercept |= type; + + if (tsk->syscall_intercept) + set_tsk_thread_flag(tsk, TIF_SYSCALL_INTERCEPT); +} + +static inline void __clear_tsk_syscall_intercept(struct task_struct *tsk, + unsigned int type) +{ + tsk->syscall_intercept &= ~type; + + if (tsk->syscall_intercept == 0) + clear_tsk_thread_flag(tsk, TIF_SYSCALL_INTERCEPT); +} + +static inline void set_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ + spin_lock_irq(&tsk->sighand->siglock); + __set_tsk_syscall_intercept(tsk, type); + spin_unlock_irq(&tsk->sighand->siglock); +} + +static inline void clear_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ + spin_lock_irq(&tsk->sighand->siglock); + __clear_tsk_syscall_intercept(tsk, type); + spin_unlock_irq(&tsk->sighand->siglock); +} + +#else +static inline void __set_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ +} +static inline void set_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ +} +static inline void __clear_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ +} +static inline void clear_tsk_syscall_intercept(struct task_struct *tsk, unsigned int type) +{ +} +#endif + +#endif + diff --git a/kernel/fork.c b/kernel/fork.c index 76d3f3387554..27c8cab27a11 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -49,7 +49,7 @@ #include <linux/cgroup.h> #include <linux/security.h> #include <linux/hugetlb.h> -#include <linux/seccomp.h> +#include <linux/syscall_intercept.h> #include <linux/swap.h> #include <linux/syscalls.h> #include <linux/jiffies.h> @@ -920,6 +920,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) * the usage counts on the error path calling free_task. */ tsk->seccomp.filter = NULL; + tsk->syscall_intercept = 0; #endif
setup_thread_stack(tsk, orig); @@ -1642,9 +1643,14 @@ static void copy_seccomp(struct task_struct *p) * If the parent gained a seccomp mode after copying thread * flags and between before we held the sighand lock, we have * to manually enable the seccomp thread flag here. + * + * In addition current sighand lock is asserted, so it is safe + * to use the unlocked version of set_tsk_syscall_intercept. */ if (p->seccomp.mode != SECCOMP_MODE_DISABLED) - set_tsk_thread_flag(p, TIF_SECCOMP); + __set_tsk_syscall_intercept(p, SYSINT_SECCOMP); + else + __clear_tsk_syscall_intercept(p, SYSINT_SECCOMP); #endif }
diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 3ee59ce0a323..d0643b500f2e 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -28,6 +28,7 @@ #include <linux/slab.h> #include <linux/syscalls.h> #include <linux/sysctl.h> +#include <linux/syscall_intercept.h>
#ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER #include <asm/syscall.h> @@ -352,14 +353,14 @@ static inline void seccomp_assign_mode(struct task_struct *task,
task->seccomp.mode = seccomp_mode; /* - * Make sure TIF_SECCOMP cannot be set before the mode (and + * Make sure SYSINT_SECCOMP cannot be set before the mode (and * filter) is set. */ smp_mb__before_atomic(); /* Assume default seccomp processes want spec flaw mitigation. */ if ((flags & SECCOMP_FILTER_FLAG_SPEC_ALLOW) == 0) arch_seccomp_spec_mitigate(task); - set_tsk_thread_flag(task, TIF_SECCOMP); + __set_tsk_syscall_intercept(task, SYSINT_SECCOMP); }
#ifdef CONFIG_SECCOMP_FILTER @@ -925,7 +926,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
/* * Make sure that any changes to mode from another thread have - * been seen after TIF_SECCOMP was seen. + * been seen after SYSINT_SECCOMP was seen. */ rmb();
Syscalls that use common entry code (x86 at the moment of this writing) need to have their defines updated inside this commit. This added a measureable overhead of 1ns to seccomp_benchmark selftests on a bare-metal AMD system.
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- arch/x86/include/asm/thread_info.h | 4 ++-- include/linux/entry-common.h | 6 +----- kernel/entry/common.c | 24 +++++++++++++++++++++--- 3 files changed, 24 insertions(+), 10 deletions(-)
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h index 267701ae3d86..cf723181e1f2 100644 --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -82,7 +82,7 @@ struct thread_info { #define TIF_SSBD 5 /* Speculative store bypass disable */ #define TIF_SYSCALL_EMU 6 /* syscall emulation active */ #define TIF_SYSCALL_AUDIT 7 /* syscall auditing active */ -#define TIF_SECCOMP 8 /* secure computing */ +#define TIF_SYSCALL_INTERCEPT 8 /* Intercept system call */ #define TIF_SPEC_IB 9 /* Indirect branch speculation mitigation */ #define TIF_SPEC_FORCE_UPDATE 10 /* Force speculation MSR update in context switch */ #define TIF_USER_RETURN_NOTIFY 11 /* notify kernel of userspace return */ @@ -112,7 +112,7 @@ struct thread_info { #define _TIF_SSBD (1 << TIF_SSBD) #define _TIF_SYSCALL_EMU (1 << TIF_SYSCALL_EMU) #define _TIF_SYSCALL_AUDIT (1 << TIF_SYSCALL_AUDIT) -#define _TIF_SECCOMP (1 << TIF_SECCOMP) +#define _TIF_SYSCALL_INTERCEPT (1 << TIF_SYSCALL_INTERCEPT) #define _TIF_SPEC_IB (1 << TIF_SPEC_IB) #define _TIF_SPEC_FORCE_UPDATE (1 << TIF_SPEC_FORCE_UPDATE) #define _TIF_USER_RETURN_NOTIFY (1 << TIF_USER_RETURN_NOTIFY) diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h index efebbffcd5cc..72ce9ca860c6 100644 --- a/include/linux/entry-common.h +++ b/include/linux/entry-common.h @@ -21,10 +21,6 @@ # define _TIF_SYSCALL_TRACEPOINT (0) #endif
-#ifndef _TIF_SECCOMP -# define _TIF_SECCOMP (0) -#endif - #ifndef _TIF_SYSCALL_AUDIT # define _TIF_SYSCALL_AUDIT (0) #endif @@ -45,7 +41,7 @@ #endif
#define SYSCALL_ENTER_WORK \ - (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | _TIF_SECCOMP | \ + (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | _TIF_SYSCALL_INTERCEPT | \ _TIF_SYSCALL_TRACEPOINT | _TIF_SYSCALL_EMU | \ ARCH_SYSCALL_ENTER_WORK)
diff --git a/kernel/entry/common.c b/kernel/entry/common.c index 9852e0d62d95..80db3f146462 100644 --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -4,6 +4,7 @@ #include <linux/entry-common.h> #include <linux/livepatch.h> #include <linux/audit.h> +#include <linux/syscall_intercept.h>
#define CREATE_TRACE_POINTS #include <trace/events/syscalls.h> @@ -41,6 +42,20 @@ static inline void syscall_enter_audit(struct pt_regs *regs, long syscall) } }
+static inline long do_syscall_intercept(struct pt_regs *regs) +{ + int sysint_work = READ_ONCE(current->syscall_intercept); + int ret; + + if (sysint_work & SYSINT_SECCOMP) { + ret = __secure_computing(NULL); + if (ret == -1L) + return ret; + } + + return 0; +} + static long syscall_trace_enter(struct pt_regs *regs, long syscall, unsigned long ti_work) { @@ -53,9 +68,12 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall, return -1L; }
- /* Do seccomp after ptrace, to catch any tracer changes. */ - if (ti_work & _TIF_SECCOMP) { - ret = __secure_computing(NULL); + /* + * Do syscall interception like seccomp after ptrace, to catch + * any tracer changes. + */ + if (ti_work & _TIF_SYSCALL_INTERCEPT) { + ret = do_syscall_intercept(regs); if (ret == -1L) return ret; }
Syscall user redirection requires the signal trampoline code to not be captured, in order to support returning with a locked selector while avoiding recursion back into the signal handler. For ia-32, which has the trampoline in the vDSO, expose the entry points to the kernel, such that it can avoid dispatching syscalls from that region to userspace.
Suggested-by: Andy Lutomirski luto@amacapital.net Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- arch/x86/entry/vdso/vdso2c.c | 2 ++ arch/x86/entry/vdso/vdso32/sigreturn.S | 2 ++ arch/x86/entry/vdso/vma.c | 15 +++++++++++++++ arch/x86/include/asm/elf.h | 1 + arch/x86/include/asm/vdso.h | 2 ++ 5 files changed, 22 insertions(+)
diff --git a/arch/x86/entry/vdso/vdso2c.c b/arch/x86/entry/vdso/vdso2c.c index 7380908045c7..2d0f3d8bcc25 100644 --- a/arch/x86/entry/vdso/vdso2c.c +++ b/arch/x86/entry/vdso/vdso2c.c @@ -101,6 +101,8 @@ struct vdso_sym required_syms[] = { {"__kernel_sigreturn", true}, {"__kernel_rt_sigreturn", true}, {"int80_landing_pad", true}, + {"vdso32_rt_sigreturn_landing_pad", true}, + {"vdso32_sigreturn_landing_pad", true}, };
__attribute__((format(printf, 1, 2))) __attribute__((noreturn)) diff --git a/arch/x86/entry/vdso/vdso32/sigreturn.S b/arch/x86/entry/vdso/vdso32/sigreturn.S index c3233ee98a6b..1bd068f72d4c 100644 --- a/arch/x86/entry/vdso/vdso32/sigreturn.S +++ b/arch/x86/entry/vdso/vdso32/sigreturn.S @@ -18,6 +18,7 @@ __kernel_sigreturn: movl $__NR_sigreturn, %eax SYSCALL_ENTER_KERNEL .LEND_sigreturn: +SYM_INNER_LABEL(vdso32_sigreturn_landing_pad, SYM_L_GLOBAL) nop .size __kernel_sigreturn,.-.LSTART_sigreturn
@@ -29,6 +30,7 @@ __kernel_rt_sigreturn: movl $__NR_rt_sigreturn, %eax SYSCALL_ENTER_KERNEL .LEND_rt_sigreturn: +SYM_INNER_LABEL(vdso32_rt_sigreturn_landing_pad, SYM_L_GLOBAL) nop .size __kernel_rt_sigreturn,.-.LSTART_rt_sigreturn .previous diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c index 9185cb1d13b9..1828ed2f4a5f 100644 --- a/arch/x86/entry/vdso/vma.c +++ b/arch/x86/entry/vdso/vma.c @@ -436,6 +436,21 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp) } #endif
+int arch_syscall_is_vdso_sigreturn(struct pt_regs *regs) +{ +#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) + const struct vdso_image *image = current->mm->context.vdso_image; + unsigned long vdso = (unsigned long) current->mm->context.vdso; + + if (in_ia32_syscall() && image == &vdso_image_32) { + if (regs->ip == vdso + image->sym_vdso32_sigreturn_landing_pad || + regs->ip == vdso + image->sym_vdso32_rt_sigreturn_landing_pad) + return 1; + } +#endif + return 0; +} + #ifdef CONFIG_X86_64 static __init int vdso_setup(char *s) { diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index 452beed7892b..b8fd74f7207d 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -385,6 +385,7 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp); extern int compat_arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp); +extern int arch_syscall_is_vdso_sigreturn(struct pt_regs *regs); #define compat_arch_setup_additional_pages compat_arch_setup_additional_pages
/* Do not change the values. See get_align_mask() */ diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h index bbcdc7b8f963..589f489dd375 100644 --- a/arch/x86/include/asm/vdso.h +++ b/arch/x86/include/asm/vdso.h @@ -27,6 +27,8 @@ struct vdso_image { long sym___kernel_rt_sigreturn; long sym___kernel_vsyscall; long sym_int80_landing_pad; + long sym_vdso32_sigreturn_landing_pad; + long sym_vdso32_rt_sigreturn_landing_pad; };
#ifdef CONFIG_X86_64
On Mon, Aug 10, 2020 at 4:26 PM Gabriel Krisman Bertazi krisman@collabora.com wrote:
Syscall user redirection requires the signal trampoline code to not be captured, in order to support returning with a locked selector while avoiding recursion back into the signal handler. For ia-32, which has the trampoline in the vDSO, expose the entry points to the kernel, such that it can avoid dispatching syscalls from that region to userspace.
Almost looks good.
+int arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
If you make that bool instead of int, feel free to add:
Acked-by: Andy Lutomirski luto@kernel.org
SYS_USER_DISPATCH will be triggered when a syscall is sent to userspace by the Syscall User Dispatch mechanism. This adjusts eventual BUILD_BUG_ON around the tree.
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- arch/x86/kernel/signal_compat.c | 2 +- include/uapi/asm-generic/siginfo.h | 3 ++- 2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c index 9ccbf0576cd0..210aecc6eab9 100644 --- a/arch/x86/kernel/signal_compat.c +++ b/arch/x86/kernel/signal_compat.c @@ -31,7 +31,7 @@ static inline void signal_compat_build_tests(void) BUILD_BUG_ON(NSIGBUS != 5); BUILD_BUG_ON(NSIGTRAP != 5); BUILD_BUG_ON(NSIGCHLD != 6); - BUILD_BUG_ON(NSIGSYS != 1); + BUILD_BUG_ON(NSIGSYS != 2);
/* This is part of the ABI and can never change in size: */ BUILD_BUG_ON(sizeof(compat_siginfo_t) != 128); diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h index cb3d6c267181..37741908b846 100644 --- a/include/uapi/asm-generic/siginfo.h +++ b/include/uapi/asm-generic/siginfo.h @@ -284,7 +284,8 @@ typedef struct siginfo { * SIGSYS si_codes */ #define SYS_SECCOMP 1 /* seccomp triggered */ -#define NSIGSYS 1 +#define SYS_USER_DISPATCH 2 /* syscall user dispatch triggered */ +#define NSIGSYS 2
/* * SIGEMT si_codes
Introduce a mechanism to quickly disable/enable syscall handling for a specific process and redirect to userspace via SIGSYS. This is useful for processes with parts that require syscall redirection and parts that don't, but who need to perform this boundary crossing really fast, without paying the cost of a system call to reconfigure syscall handling on each boundary transition. This is particularly important for Windows games running over Wine.
The proposed interface looks like this:
prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <start_addr>, <end_addr>, [selector])
The range [<start_addr>,<end_addr>] is a part of the process memory map that is allowed to by-pass the redirection code and dispatch syscalls directly, such that in fast paths a process doesn't need to disable the trap nor the kernel has to check the selector. This is essential to return from SIGSYS to a blocked area without triggering another SIGSYS from rt_sigreturn.
selector is an optional pointer to a char-sized userspace memory region that has a key switch for the mechanism. This key switch is set to either PR_SYS_DISPATCH_ON, PR_SYS_DISPATCH_OFF to enable and disable the redirection without calling the kernel.
The feature is meant to be set per-thread and it is disabled on fork/clone/execv.
Internally, this doesn't add overhead to the syscall hot path, and it requires very little per-architecture support. I avoided using seccomp, even though it duplicates some functionality, due to previous feedback that maybe it shouldn't mix with seccomp since it is not a security mechanism. And obviously, this should never be considered a security mechanism, since any part of the program can by-pass it by using the syscall dispatcher.
For the sysinfo benchmark, which measures the overhead added to executing a native syscall that doesn't require interception, the overhead using only the direct dispatcher region to issue syscalls is pretty much irrelevant. The overhead of using the selector goes around 40ns for a native (unredirected) syscall in my system, and it is (as expected) dominated by the supervisor-mode user-address access. In fact, with SMAP off, the overhead is consistently less than 5ns on my test box.
An example code using this interface can be found at: https://gitlab.collabora.com/krisman/syscall-disable-personality
Changes since v4: (Andy Lutomirski) - Allow sigreturn coming from vDSO - Exit with SIGSYS instead of SIGSEGV on bad selector (Thomas Gleixner) - Use sizeof selector in access_ok - Document usage of __get_user - Use constant for state value - Split out x86 parts - Rebase on top of Gleixner's common entry code - Don't expose do_syscall_user_dispatch
Changes since v3: - NTR.
Changes since v2: (Matthew Wilcox suggestions) - Drop __user on non-ptr type. - Move #define closer to similar defs - Allow a memory region that can dispatch directly (Kees Cook suggestions) - Improve kconfig summary line - Move flag cleanup on execve to begin_new_exec - Hint branch predictor in the syscall path (Me) - Convert selector to char
Changes since RFC: (Kees Cook suggestions) - Don't mention personality while explaining the feature - Use syscall_get_nr - Remove header guard on several places - Convert WARN_ON to WARN_ON_ONCE - Explicit check for state values - Rename to syscall user dispatcher
Cc: Matthew Wilcox willy@infradead.org Cc: Andy Lutomirski luto@kernel.org Cc: Paul Gofman gofmanp@gmail.com Cc: Kees Cook keescook@chromium.org Cc: linux-api@vger.kernel.org Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- arch/Kconfig | 21 ++++++ fs/exec.c | 8 ++ include/linux/sched.h | 2 + include/linux/syscall_intercept.h | 1 + include/linux/syscall_user_dispatch.h | 29 ++++++++ include/uapi/linux/prctl.h | 5 ++ kernel/entry/Makefile | 1 + kernel/entry/common.h | 15 ++++ kernel/entry/syscall_user_dispatch.c | 101 ++++++++++++++++++++++++++ kernel/sys.c | 5 ++ 10 files changed, 188 insertions(+) create mode 100644 include/linux/syscall_user_dispatch.h create mode 100644 kernel/entry/common.h create mode 100644 kernel/entry/syscall_user_dispatch.c
diff --git a/arch/Kconfig b/arch/Kconfig index a1124481d910..76d8ccedaead 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -468,6 +468,27 @@ config SECCOMP_FILTER
See Documentation/userspace-api/seccomp_filter.rst for details.
+config HAVE_ARCH_SYSCALL_USER_DISPATCH + bool + help + An arch should select this symbol if it provides all of these things: + - TIF_SYSCALL_INTERCEPT + - syscall_get_arch + - syscall_rollback + - syscall_get_nr + - SIGSYS siginfo_t support + - arch_syscall_is_vdso_sigreturn + +config SYSCALL_USER_DISPATCH + bool "Support syscall redirection to userspace dispatcher" + depends on HAVE_ARCH_SYSCALL_USER_DISPATCH + help + Enable tasks to ask the kernel to redirect syscalls not + issued from a predefined dispatcher back to userspace, + depending on a userspace memory selector. + + This option is useful to optimize games running over Wine. + config HAVE_ARCH_STACKLEAK bool help diff --git a/fs/exec.c b/fs/exec.c index 3698252719a3..57ca30dd0623 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -62,6 +62,7 @@ #include <linux/oom.h> #include <linux/compat.h> #include <linux/vmalloc.h> +#include <linux/syscall_intercept.h>
#include <linux/uaccess.h> #include <asm/mmu_context.h> @@ -1408,6 +1409,13 @@ int begin_new_exec(struct linux_binprm * bprm) flush_thread(); me->personality &= ~bprm->per_clear;
+ /* + * Prevent Syscall User Dispatch from crossing application + * boundaries. sighand is already unshared, so it is safe to + * use the unlocked version here. + */ + __clear_tsk_syscall_intercept(me, SYSINT_USER_DISPATCH); + /* * We have to apply CLOEXEC before we change whether the process is * dumpable (in setup_new_exec) to avoid a race with a process in userspace diff --git a/include/linux/sched.h b/include/linux/sched.h index 23b3e155ccab..b4ee5206ff37 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -33,6 +33,7 @@ #include <linux/posix-timers.h> #include <linux/rseq.h> #include <linux/kcsan.h> +#include <linux/syscall_user_dispatch.h>
/* task_struct member predeclarations (sorted alphabetically): */ struct audit_context; @@ -958,6 +959,7 @@ struct task_struct { struct { unsigned int syscall_intercept; struct seccomp seccomp; + struct syscall_user_dispatch syscall_dispatch; };
/* Thread group tracking: */ diff --git a/include/linux/syscall_intercept.h b/include/linux/syscall_intercept.h index 725d157699da..21bc2eb668f3 100644 --- a/include/linux/syscall_intercept.h +++ b/include/linux/syscall_intercept.h @@ -10,6 +10,7 @@ #include <linux/thread_info.h>
#define SYSINT_SECCOMP 0x1 +#define SYSINT_USER_DISPATCH 0x2
#ifdef TIF_SYSCALL_INTERCEPT
diff --git a/include/linux/syscall_user_dispatch.h b/include/linux/syscall_user_dispatch.h new file mode 100644 index 000000000000..f831358bfaab --- /dev/null +++ b/include/linux/syscall_user_dispatch.h @@ -0,0 +1,29 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright (C) 2020 Collabora Ltd. + */ +#ifndef _SYSCALL_USER_DISPATCH_H +#define _SYSCALL_USER_DISPATCH_H + +#ifdef CONFIG_SYSCALL_USER_DISPATCH +struct syscall_user_dispatch { + char __user *selector; + unsigned long dispatcher_start; + unsigned long dispatcher_end; +}; + +int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start, + unsigned long dispatcher_end, + char __user *selector); +#else +struct syscall_user_dispatch {}; + +static inline int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start, + unsigned long dispatcher_end, + char __user *selector) +{ + return -EINVAL; +} +#endif /* CONFIG_SYSCALL_USER_DISPATCH */ + +#endif /* _SYSCALL_USER_DISPATCH_H */ diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 07b4f8131e36..96265246383d 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -238,4 +238,9 @@ struct prctl_mm_map { #define PR_SET_IO_FLUSHER 57 #define PR_GET_IO_FLUSHER 58
+/* Dispatch syscalls to a userspace handler */ +#define PR_SET_SYSCALL_USER_DISPATCH 59 +# define PR_SYS_DISPATCH_OFF 0 +# define PR_SYS_DISPATCH_ON 1 + #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/entry/Makefile b/kernel/entry/Makefile index 34c8a3f1c735..81182ab5a40c 100644 --- a/kernel/entry/Makefile +++ b/kernel/entry/Makefile @@ -11,3 +11,4 @@ CFLAGS_common.o += -fno-stack-protector
obj-$(CONFIG_GENERIC_ENTRY) += common.o obj-$(CONFIG_KVM_XFER_TO_GUEST_WORK) += kvm.o +obj-$(CONFIG_SYSCALL_USER_DISPATCH) += syscall_user_dispatch.o diff --git a/kernel/entry/common.h b/kernel/entry/common.h new file mode 100644 index 000000000000..557ecaa3fb31 --- /dev/null +++ b/kernel/entry/common.h @@ -0,0 +1,15 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _COMMON_H +#define _COMMON_H + +#ifdef CONFIG_SYSCALL_USER_DISPATCH +int do_syscall_user_dispatch(struct pt_regs *regs); +#else +static inline int do_syscall_user_dispatch(struct pt_regs *regs) +{ + WARN_ON_ONCE(1); + return 0; +} +#endif /* CONFIG_SYSCALL_USER_DISPATCH */ + +#endif diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c new file mode 100644 index 000000000000..12ea01711dc7 --- /dev/null +++ b/kernel/entry/syscall_user_dispatch.c @@ -0,0 +1,101 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright (C) 2020 Collabora Ltd. + */ +#include <linux/sched.h> +#include <linux/prctl.h> +#include <linux/syscall_intercept.h> +#include <linux/syscall_user_dispatch.h> +#include <linux/uaccess.h> +#include <linux/signal.h> +#include <linux/elf.h> + +#include <asm/syscall.h> + +#include <linux/sched/signal.h> +#include <linux/sched/task_stack.h> + +static void trigger_sigsys(struct pt_regs *regs) +{ + struct kernel_siginfo info; + + clear_siginfo(&info); + info.si_signo = SIGSYS; + info.si_code = SYS_USER_DISPATCH; + info.si_call_addr = (void __user *)KSTK_EIP(current); + info.si_errno = 0; + info.si_arch = syscall_get_arch(current); + info.si_syscall = syscall_get_nr(current, regs); + + force_sig_info(&info); +} + +int do_syscall_user_dispatch(struct pt_regs *regs) +{ + struct syscall_user_dispatch *sd = ¤t->syscall_dispatch; + unsigned long ip = instruction_pointer(regs); + char state; + + if (likely(ip >= sd->dispatcher_start && ip <= sd->dispatcher_end)) + return 0; + + if (unlikely(arch_syscall_is_vdso_sigreturn(regs))) + return 0; + + if (likely(sd->selector)) { + /* + * access_ok() is performed once, at prctl time, when + * the selector is loaded by userspace. + */ + if (unlikely(__get_user(state, sd->selector))) + do_exit(SIGSEGV); + + if (likely(state == PR_SYS_DISPATCH_OFF)) + return 0; + + if (state != PR_SYS_DISPATCH_ON) + do_exit(SIGSYS); + } + + syscall_rollback(current, regs); + trigger_sigsys(regs); + + return 1; +} + +int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start, + unsigned long dispatcher_end, char __user *selector) +{ + switch (mode) { + case PR_SYS_DISPATCH_OFF: + if (dispatcher_start || dispatcher_end || selector) + return -EINVAL; + break; + case PR_SYS_DISPATCH_ON: + /* + * Validate the direct dispatcher region just for basic + * sanity. If the user is able to submit a syscall from + * an address, that address is obviously valid. + */ + if (dispatcher_end < dispatcher_start) + return -EINVAL; + + if (selector && !access_ok(selector, sizeof(*selector))) + return -EFAULT; + + break; + default: + return -EINVAL; + } + + current->syscall_dispatch.selector = selector; + current->syscall_dispatch.dispatcher_start = dispatcher_start; + current->syscall_dispatch.dispatcher_end = dispatcher_end; + + if (mode == PR_SYS_DISPATCH_ON) + set_tsk_syscall_intercept(current, SYSINT_USER_DISPATCH); + else + clear_tsk_syscall_intercept(current, SYSINT_USER_DISPATCH); + + return 0; +} diff --git a/kernel/sys.c b/kernel/sys.c index ca11af9d815d..f7ed56177f07 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -42,6 +42,7 @@ #include <linux/syscore_ops.h> #include <linux/version.h> #include <linux/ctype.h> +#include <linux/syscall_user_dispatch.h>
#include <linux/compat.h> #include <linux/syscalls.h> @@ -2530,6 +2531,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER; break; + case PR_SET_SYSCALL_USER_DISPATCH: + error = set_syscall_user_dispatch((int) arg2, arg3, arg4, + (char __user *) arg5); + break; default: error = -EINVAL; break;
Syscall User Dispatch (SUD) must take precedence over seccomp, since the use case is emulation (it can be invoked with a different ABI) such that seccomp filtering by syscall number doesn't make sense in the first place. In addition, either the syscall is dispatched back to userspace, in which case there is no resource for seccomp to protect, or the syscall will be executed, and seccomp will execute next.
Regarding ptrace, I experimented with before and after, and while the same ABI argument applies, I felt it was easier to debug if I let ptrace happen for syscalls that are dispatched back to userspace. In addition, doing it after ptrace makes the code in syscall_exit_work slightly simpler, since it doesn't require special handling for this feature.
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- kernel/entry/common.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/kernel/entry/common.c b/kernel/entry/common.c index 80db3f146462..f05a4ee094a1 100644 --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -6,6 +6,8 @@ #include <linux/audit.h> #include <linux/syscall_intercept.h>
+#include "common.h" + #define CREATE_TRACE_POINTS #include <trace/events/syscalls.h>
@@ -47,6 +49,12 @@ static inline long do_syscall_intercept(struct pt_regs *regs) int sysint_work = READ_ONCE(current->syscall_intercept); int ret;
+ if (sysint_work & SYSINT_USER_DISPATCH) { + ret = do_syscall_user_dispatch(regs); + if (ret == -1L) + return ret; + } + if (sysint_work & SYSINT_SECCOMP) { ret = __secure_computing(NULL); if (ret == -1L)
Syscall User Dispatch requirements are fully supported in x86. This patch flips the switch, marking it as supported. This was tested against Syscall User Dispatch selftest.
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- arch/x86/Kconfig | 1 + 1 file changed, 1 insertion(+)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 9a2849527dd7..431440566888 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -150,6 +150,7 @@ config X86 select HAVE_ARCH_COMPAT_MMAP_BASES if MMU && COMPAT select HAVE_ARCH_PREL32_RELOCATIONS select HAVE_ARCH_SECCOMP_FILTER + select HAVE_ARCH_SYSCALL_USER_DISPATCH select HAVE_ARCH_THREAD_STRUCT_WHITELIST select HAVE_ARCH_STACKLEAK select HAVE_ARCH_TRACEHOOK
Implement functionality tests for syscall user dispatch. In order to make the test portable, refrain from open coding syscall dispatchers and calculating glibc memory ranges.
Changes since v4: - Update bad selector test to reflect change in API
Changes since v3: - Sort entry in Makefile - Add SPDX header - Use __NR_syscalls if available
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- tools/testing/selftests/Makefile | 1 + .../syscall_user_dispatch/.gitignore | 2 + .../selftests/syscall_user_dispatch/Makefile | 9 + .../selftests/syscall_user_dispatch/config | 1 + .../syscall_user_dispatch.c | 292 ++++++++++++++++++ 5 files changed, 305 insertions(+) create mode 100644 tools/testing/selftests/syscall_user_dispatch/.gitignore create mode 100644 tools/testing/selftests/syscall_user_dispatch/Makefile create mode 100644 tools/testing/selftests/syscall_user_dispatch/config create mode 100644 tools/testing/selftests/syscall_user_dispatch/syscall_user_dispatch.c
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile index e03bc15ce731..1521daffa4c3 100644 --- a/tools/testing/selftests/Makefile +++ b/tools/testing/selftests/Makefile @@ -55,6 +55,7 @@ TARGETS += sparc64 TARGETS += splice TARGETS += static_keys TARGETS += sync +TARGETS += syscall_user_dispatch TARGETS += sysctl TARGETS += tc-testing TARGETS += timens diff --git a/tools/testing/selftests/syscall_user_dispatch/.gitignore b/tools/testing/selftests/syscall_user_dispatch/.gitignore new file mode 100644 index 000000000000..637f08107add --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/.gitignore @@ -0,0 +1,2 @@ +# SPDX-License-Identifier: GPL-2.0-only +syscall_user_dispatch diff --git a/tools/testing/selftests/syscall_user_dispatch/Makefile b/tools/testing/selftests/syscall_user_dispatch/Makefile new file mode 100644 index 000000000000..eeb07a791057 --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/Makefile @@ -0,0 +1,9 @@ +# SPDX-License-Identifier: GPL-2.0 +top_srcdir = ../../../.. +INSTALL_HDR_PATH = $(top_srcdir)/usr +LINUX_HDR_PATH = $(INSTALL_HDR_PATH)/include/ + +CFLAGS += -Wall -I$(LINUX_HDR_PATH) + +TEST_GEN_PROGS := syscall_user_dispatch +include ../lib.mk diff --git a/tools/testing/selftests/syscall_user_dispatch/config b/tools/testing/selftests/syscall_user_dispatch/config new file mode 100644 index 000000000000..22c4dfe167ca --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/config @@ -0,0 +1 @@ +CONFIG_SYSCALL_USER_DISPATCH=y diff --git a/tools/testing/selftests/syscall_user_dispatch/syscall_user_dispatch.c b/tools/testing/selftests/syscall_user_dispatch/syscall_user_dispatch.c new file mode 100644 index 000000000000..885b5125bd90 --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/syscall_user_dispatch.c @@ -0,0 +1,292 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Copyright (c) 2020 Collabora Ltd. + * + * Test code for syscall user dispatch + */ + +#define _GNU_SOURCE +#include <sys/prctl.h> +#include <sys/sysinfo.h> +#include <sys/syscall.h> +#include <signal.h> + +#include <asm/unistd.h> +#include "../kselftest_harness.h" + +#ifndef PR_SET_SYSCALL_USER_DISPATCH +# define PR_SET_SYSCALL_USER_DISPATCH 59 +# define PR_SYS_DISPATCH_OFF 0 +# define PR_SYS_DISPATCH_ON 1 +#endif + +#ifndef SYS_USER_DISPATCH +# define SYS_USER_DISPATCH 2 +#endif + +#ifdef __NR_syscalls +# define MAGIC_SYSCALL_1 (__NR_syscalls + 1) /* Bad Linux syscall number */ +#else +# define MAGIC_SYSCALL_1 (0xff00) /* Bad Linux syscall number */ +#endif + +#define SYSCALL_DISPATCH_ON(x) ((x) = 1) +#define SYSCALL_DISPATCH_OFF(x) ((x) = 0) + +/* Test Summary: + * + * - dispatch_trigger_sigsys: Verify if PR_SET_SYSCALL_USER_DISPATCH is + * able to trigger SIGSYS on a syscall. + * + * - bad_selector: Test that a bad selector value triggers SIGSYS with + * si_errno EINVAL. + * + * - bad_prctl_param: Test that the API correctly rejects invalid + * parameters on prctl + * + * - dispatch_and_return: Test that a syscall is selectively dispatched + * to userspace depending on the value of selector. + * + * - disable_dispatch: Test that the PR_SYS_DISPATCH_OFF correctly + * disables the dispatcher + * + * - direct_dispatch_range: Test that a syscall within the allowed range + * can bypass the dispatcher. + */ + +TEST_SIGNAL(dispatch_trigger_sigsys, SIGSYS) +{ + char sel = 0; + struct sysinfo info; + int ret; + + ret = sysinfo(&info); + ASSERT_EQ(0, ret); + + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 0, 0, &sel); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support CONFIG_SYSCALL_USER_DISPATCH"); + } + + SYSCALL_DISPATCH_ON(sel); + + sysinfo(&info); + + EXPECT_FALSE(true) { + TH_LOG("Unreachable!"); + } +} + +TEST(bad_prctl_param) +{ + char sel = 0; + int op; + + /* Invalid op */ + op = -1; + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0, 0, &sel); + ASSERT_EQ(EINVAL, errno); + + /* PR_SYS_DISPATCH_OFF */ + op = PR_SYS_DISPATCH_OFF; + + /* start_addr != 0 */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0xff, 0); + EXPECT_EQ(EINVAL, errno); + + /* end_addr != 0 */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0xff, 0); + EXPECT_EQ(EINVAL, errno); + + /* sel != NULL */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x0, &sel); + EXPECT_EQ(EINVAL, errno); + + /* Valid parameter */ + errno = 0; + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x0, 0x0); + EXPECT_EQ(0, errno); + + /* PR_SYS_DISPATCH_ON */ + op = PR_SYS_DISPATCH_ON; + + /* start_addr > end_addr */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0x0, &sel); + EXPECT_EQ(EINVAL, errno); + + /* Invalid selector */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x1, (void *) -1); + ASSERT_EQ(EFAULT, errno); +} + +/* + * Use global selector for handle_sigsys tests, to avoid passing + * selector to signal handler + */ +char glob_sel; +int nr_syscalls_emulated; +int si_code; +int si_errno; + +static void handle_sigsys(int sig, siginfo_t *info, void *ucontext) +{ + si_code = info->si_code; + si_errno = info->si_errno; + + if (info->si_syscall == MAGIC_SYSCALL_1) + nr_syscalls_emulated++; + + /* In preparation for sigreturn. */ + SYSCALL_DISPATCH_OFF(glob_sel); +} + +TEST(dispatch_and_return) +{ + long ret; + struct sigaction act; + sigset_t mask; + + glob_sel = 0; + nr_syscalls_emulated = 0; + si_code = 0; + si_errno = 0; + + memset(&act, 0, sizeof(act)); + sigemptyset(&mask); + + act.sa_sigaction = handle_sigsys; + act.sa_flags = SA_SIGINFO; + act.sa_mask = mask; + + ret = sigaction(SIGSYS, &act, NULL); + ASSERT_EQ(0, ret); + + /* Make sure selector is good prior to prctl. */ + SYSCALL_DISPATCH_OFF(glob_sel); + + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 0, 0, &glob_sel); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support CONFIG_SYSCALL_USER_DISPATCH"); + } + + /* MAGIC_SYSCALL_1 doesn't exist. */ + SYSCALL_DISPATCH_OFF(glob_sel); + ret = syscall(MAGIC_SYSCALL_1); + EXPECT_EQ(-1, ret) { + TH_LOG("Dispatch triggered unexpectedly"); + } + + /* MAGIC_SYSCALL_1 should be emulated. */ + nr_syscalls_emulated = 0; + SYSCALL_DISPATCH_ON(glob_sel); + + ret = syscall(MAGIC_SYSCALL_1); + EXPECT_EQ(MAGIC_SYSCALL_1, ret) { + TH_LOG("Failed to intercept syscall"); + } + EXPECT_EQ(1, nr_syscalls_emulated) { + TH_LOG("Failed to emulate syscall"); + } + ASSERT_EQ(SYS_USER_DISPATCH, si_code) { + TH_LOG("Bad si_code in SIGSYS"); + } + ASSERT_EQ(0, si_errno) { + TH_LOG("Bad si_errno in SIGSYS"); + } +} + +TEST_SIGNAL(bad_selector, SIGSYS) +{ + long ret; + struct sigaction act; + sigset_t mask; + struct sysinfo info; + + glob_sel = 0; + nr_syscalls_emulated = 0; + si_code = 0; + si_errno = 0; + + memset(&act, 0, sizeof(act)); + sigemptyset(&mask); + + act.sa_sigaction = handle_sigsys; + act.sa_flags = SA_SIGINFO; + act.sa_mask = mask; + + ret = sigaction(SIGSYS, &act, NULL); + ASSERT_EQ(0, ret); + + /* Make sure selector is good prior to prctl. */ + SYSCALL_DISPATCH_OFF(glob_sel); + + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 0, 0, &glob_sel); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support CONFIG_SYSCALL_USER_DISPATCH"); + } + + glob_sel = -1; + + sysinfo(&info); + + /* Even though it is ready to catch SIGSYS, the signal is + * supposed to be uncatchable. + */ + + EXPECT_FALSE(true) { + TH_LOG("Unreachable!"); + } +} + +TEST(disable_dispatch) +{ + int ret; + struct sysinfo info; + char sel = 0; + + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 0, 0, &sel); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support CONFIG_SYSCALL_USER_DISPATCH"); + } + + /* MAGIC_SYSCALL_1 doesn't exist. */ + SYSCALL_DISPATCH_OFF(glob_sel); + + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_OFF, 0, 0, 0); + EXPECT_EQ(0, ret) { + TH_LOG("Failed to unset syscall user dispatch"); + } + + /* Shouldn't have any effect... */ + SYSCALL_DISPATCH_ON(glob_sel); + + ret = syscall(__NR_sysinfo, &info); + EXPECT_EQ(0, ret) { + TH_LOG("Dispatch triggered unexpectedly"); + } +} + +TEST(direct_dispatch_range) +{ + int ret = 0; + struct sysinfo info; + char sel = 0; + + /* + * Instead of calculating libc addresses; allow the entire + * memory map and lock the selector. + */ + ret = prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 0, -1L, &sel); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support CONFIG_SYSCALL_USER_DISPATCH"); + } + + SYSCALL_DISPATCH_ON(sel); + + ret = sysinfo(&info); + ASSERT_EQ(0, ret) { + TH_LOG("Dispatch triggered unexpectedly"); + } +} + +TEST_HARNESS_MAIN
Explain the interface, provide some background and security notes.
Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com --- .../admin-guide/syscall-user-dispatch.rst | 87 +++++++++++++++++++ 1 file changed, 87 insertions(+) create mode 100644 Documentation/admin-guide/syscall-user-dispatch.rst
diff --git a/Documentation/admin-guide/syscall-user-dispatch.rst b/Documentation/admin-guide/syscall-user-dispatch.rst new file mode 100644 index 000000000000..96616660fded --- /dev/null +++ b/Documentation/admin-guide/syscall-user-dispatch.rst @@ -0,0 +1,87 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Syscall User Dispatch +===================== + +Background +---------- + +Compatibility layers like Wine need a way to efficiently emulate system +calls of only a part of their process - the part that has the +incompatible code - while being able to execute native syscalls without +a high performance penalty on the native part of the process. Seccomp +falls short on this task, since it has limited support to efficiently +filter syscalls based on memory regions, and it doesn't support removing +filters. Therefore a new mechanism is necessary. + +Syscall User Dispatch brings the filtering of the syscall dispatcher +address back to userspace. The application is in control of a flip +switch, indicating the current personality of the process. A +multiple-personality application can then flip the switch without +invoking the kernel, when crossing the compatibility layer API +boundaries, to enable/disable the syscall redirection and execute +syscalls directly (disabled) or send them to be emulated in userspace +through a SIGSYS. + +The goal of this design is to provide very quick compatibility layer +boundary crosses, which is achieved by not executing a syscall to change +personality every time the compatibility layer executes. Instead, a +userspace memory region exposed to the kernel indicates the current +personality, and the application simply modifies that variable to +configure the mechanism. + +There is a relatively high cost associated with handling signals on most +architectures, like x86, but at least for Wine, syscalls issued by +native Windows code are currently not known to be a performance problem, +since they are quite rare, at least for modern gaming applications. + +Since this mechanism is designed to capture syscalls issued by +non-native applications, it must function on syscalls whose invocation +ABI is completely unexpected to Linux. Syscall User Dispatch, therefore +doesn't rely on any of the syscall ABI to make the filtering. It uses +only the syscall dispatcher address and the userspace key. + +Interface +--------- + +A process can setup this mechanism on supported kernels +CONFIG_SYSCALL_USER_DISPATCH) by executing the following prctl: + + prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <start_addr>, <end_addr>, [selector]) + +<op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and +disable the mechanism globally for that thread. When +PR_SYS_DISPATCH_OFF is used, the other fields must be zero. + +<start_addr> and <end_addr> delimit a closed memory region interval from +which syscalls are always executed directly, regardless of the userspace +selector. This provides a fast path for the C library, which includes +the most common syscall dispatchers in the native code applications, and +also provides a way for the signal handler to return without triggering +a nested SIGSYS on (rt_)sigreturn. Users of this interface should make +sure that at least the signal trampoline code is included in this +region. In addition, for syscalls that implement the trampoline code on +the vDSO, that trampoline is never intercepted. + +[selector] is a pointer to a char-sized region in the process memory +region, that provides a quick way to enable disable syscall redirection +thread-wide, without the need to invoke the kernel directly. selector +can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF. Any other +value should terminate the program with a SIGSYS. + +Security Notes +-------------- + +Syscall User Dispatch provides functionality for compatibility layers to +quickly capture system calls issued by a non-native part of the +application, while not impacting the Linux native regions of the +process. It is not a mechanism for sandboxing system calls, and it +should not be seen as a security mechanism, since it is trivial for a +malicious application to subvert the mechanism by jumping to an allowed +dispatcher region prior to executing the syscall, or to discover the +address and modify the selector value. If the use case requires any +kind of security sandboxing, Seccomp should be used instead. + +Any fork or exec of the existing process resets the mechanism to +PR_SYS_DISPATCH_OFF.
linux-kselftest-mirror@lists.linaro.org
-
Andy Lutomirski -
Gabriel Krisman Bertazi