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@kernel.org Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com Acked-by: Andy Lutomirski luto@kernel.org Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org --- Changes since V5 - Change return address to bool (Andy) --- 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 | 2 ++ arch/x86/include/asm/vdso.h | 2 ++ 5 files changed, 23 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 50e5d3a2e70a..de60cd37070b 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

+bool 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 true; + } +#endif + return false; +} + #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 44a9b9940535..66bdfe838d61 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -388,6 +388,8 @@ extern int compat_arch_setup_additional_pages(struct linux_binprm *bprm, compat_arch_setup_additional_pages(bprm, interpreter, \ (ex->e_machine == EM_X86_64))

+extern bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs); + /* Do not change the values. See get_align_mask() */ enum align_flags { ALIGN_VA_32 = BIT(0), diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h index b5d23470f56b..98aa103eb4ab 100644 --- a/arch/x86/include/asm/vdso.h +++ b/arch/x86/include/asm/vdso.h @@ -29,6 +29,8 @@ 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

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>, <off>, <length>, [selector])

The range [<offset>,<offset>+<length>) 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.

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 Acked-by: Peter Zijlstra (Intel) peterz@infradead.org

--- Changes since v7: - Correct half-open interval in commit message (PeterZ) - Solve rebase conflicts

Changes since v6: (Matthew Wilcox) - Use unsigned long for mode (peterZ) - Change interface to {offset,len} - Use SYSCALL_WORK interface instead of TIF flags

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 --- fs/exec.c | 3 + include/linux/sched.h | 2 + include/linux/syscall_user_dispatch.h | 40 ++++++++++ include/linux/thread_info.h | 2 + include/uapi/linux/prctl.h | 5 ++ kernel/entry/Makefile | 2 +- kernel/entry/common.h | 16 ++++ kernel/entry/syscall_user_dispatch.c | 102 ++++++++++++++++++++++++++ kernel/fork.c | 1 + kernel/sys.c | 5 ++ 10 files changed, 177 insertions(+), 1 deletion(-) 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/fs/exec.c b/fs/exec.c index 547a2390baf5..aee36e5733ce 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -64,6 +64,7 @@ #include <linux/compat.h> #include <linux/vmalloc.h> #include <linux/io_uring.h> +#include <linux/syscall_user_dispatch.h>

#include <linux/uaccess.h> #include <asm/mmu_context.h> @@ -1302,6 +1303,8 @@ int begin_new_exec(struct linux_binprm * bprm) flush_thread(); me->personality &= ~bprm->per_clear;

+ clear_syscall_work_syscall_user_dispatch(me); + /* * 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 ee2fdf34095b..4b719dc2eba2 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -34,6 +34,7 @@ #include <linux/rseq.h> #include <linux/seqlock.h> #include <linux/kcsan.h> +#include <linux/syscall_user_dispatch.h> #include <asm/kmap_size.h>

/* task_struct member predeclarations (sorted alphabetically): */ @@ -1000,6 +1001,7 @@ struct task_struct { unsigned int sessionid; #endif struct seccomp seccomp; + struct syscall_user_dispatch syscall_dispatch;

/* Thread group tracking: */ u64 parent_exec_id; diff --git a/include/linux/syscall_user_dispatch.h b/include/linux/syscall_user_dispatch.h new file mode 100644 index 000000000000..9517ea16f090 --- /dev/null +++ b/include/linux/syscall_user_dispatch.h @@ -0,0 +1,40 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright (C) 2020 Collabora Ltd. + */ +#ifndef _SYSCALL_USER_DISPATCH_H +#define _SYSCALL_USER_DISPATCH_H + +#include <linux/thread_info.h> + +#ifdef CONFIG_GENERIC_ENTRY + +struct syscall_user_dispatch { + char __user *selector; + unsigned long offset; + unsigned long len; + bool on_dispatch; +}; + +int set_syscall_user_dispatch(unsigned long mode, unsigned long offset, + unsigned long len, char __user *selector); + +#define clear_syscall_work_syscall_user_dispatch(tsk) \ + clear_task_syscall_work(tsk, SYSCALL_USER_DISPATCH) + +#else +struct syscall_user_dispatch {}; + +static inline int set_syscall_user_dispatch(unsigned long mode, unsigned long offset, + unsigned long len, char __user *selector) +{ + return -EINVAL; +} + +static inline void clear_syscall_work_syscall_user_dispatch(struct task_struct *tsk) +{ +} + +#endif /* CONFIG_GENERIC_ENTRY */ + +#endif /* _SYSCALL_USER_DISPATCH_H */ diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h index ca80a214df09..c8a974cead73 100644 --- a/include/linux/thread_info.h +++ b/include/linux/thread_info.h @@ -42,6 +42,7 @@ enum syscall_work_bit { SYSCALL_WORK_BIT_SYSCALL_TRACE, SYSCALL_WORK_BIT_SYSCALL_EMU, SYSCALL_WORK_BIT_SYSCALL_AUDIT, + SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH, };

#define SYSCALL_WORK_SECCOMP BIT(SYSCALL_WORK_BIT_SECCOMP) @@ -49,6 +50,7 @@ enum syscall_work_bit { #define SYSCALL_WORK_SYSCALL_TRACE BIT(SYSCALL_WORK_BIT_SYSCALL_TRACE) #define SYSCALL_WORK_SYSCALL_EMU BIT(SYSCALL_WORK_BIT_SYSCALL_EMU) #define SYSCALL_WORK_SYSCALL_AUDIT BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT) +#define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH) #endif

#include <asm/thread_info.h> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 7f0827705c9a..90deb41c8a34 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -247,4 +247,9 @@ 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..095c775e001e 100644 --- a/kernel/entry/Makefile +++ b/kernel/entry/Makefile @@ -9,5 +9,5 @@ KCOV_INSTRUMENT := n CFLAGS_REMOVE_common.o = -fstack-protector -fstack-protector-strong CFLAGS_common.o += -fno-stack-protector

-obj-$(CONFIG_GENERIC_ENTRY) += common.o +obj-$(CONFIG_GENERIC_ENTRY) += common.o syscall_user_dispatch.o obj-$(CONFIG_KVM_XFER_TO_GUEST_WORK) += kvm.o diff --git a/kernel/entry/common.h b/kernel/entry/common.h new file mode 100644 index 000000000000..cd0c4e5f143e --- /dev/null +++ b/kernel/entry/common.h @@ -0,0 +1,16 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _COMMON_H +#define _COMMON_H + +bool do_syscall_user_dispatch(struct pt_regs *regs); + +static inline bool on_syscall_dispatch(void) +{ + if (unlikely(current->syscall_dispatch.on_dispatch)) { + current->syscall_dispatch.on_dispatch = false; + return true; + } + return false; +} + +#endif diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c new file mode 100644 index 000000000000..131c38a0b628 --- /dev/null +++ b/kernel/entry/syscall_user_dispatch.c @@ -0,0 +1,102 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright (C) 2020 Collabora Ltd. + */ +#include <linux/sched.h> +#include <linux/prctl.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); +} + +bool do_syscall_user_dispatch(struct pt_regs *regs) +{ + struct syscall_user_dispatch *sd = &current->syscall_dispatch; + char state; + + if (likely(instruction_pointer(regs) - sd->offset < sd->len)) + return false; + + if (unlikely(arch_syscall_is_vdso_sigreturn(regs))) + return false; + + 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 false; + + if (state != PR_SYS_DISPATCH_ON) + do_exit(SIGSYS); + } + + sd->on_dispatch = true; + syscall_rollback(current, regs); + trigger_sigsys(regs); + + return true; +} + +int set_syscall_user_dispatch(unsigned long mode, unsigned long offset, + unsigned long len, char __user *selector) +{ + switch (mode) { + case PR_SYS_DISPATCH_OFF: + if (offset || len || selector) + return -EINVAL; + break; + case PR_SYS_DISPATCH_ON: + /* + * Validate the direct dispatcher region just for basic + * sanity against overflow and a 0-sized dispatcher + * region. If the user is able to submit a syscall from + * an address, that address is obviously valid. + */ + if (offset && offset + len <= offset) + return -EINVAL; + + if (selector && !access_ok(selector, sizeof(*selector))) + return -EFAULT; + + break; + default: + return -EINVAL; + } + + current->syscall_dispatch.selector = selector; + current->syscall_dispatch.offset = offset; + current->syscall_dispatch.len = len; + current->syscall_dispatch.on_dispatch = false; + + if (mode == PR_SYS_DISPATCH_ON) + set_syscall_work(SYSCALL_USER_DISPATCH); + else + clear_syscall_work(SYSCALL_USER_DISPATCH); + + return 0; +} diff --git a/kernel/fork.c b/kernel/fork.c index 9a01b89ed05c..99c76dab31c1 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -906,6 +906,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) clear_user_return_notifier(tsk); clear_tsk_need_resched(tsk); set_task_stack_end_magic(tsk); + clear_syscall_work_syscall_user_dispatch(tsk);

#ifdef CONFIG_STACKPROTECTOR tsk->stack_canary = get_random_canary(); diff --git a/kernel/sys.c b/kernel/sys.c index a730c03ee607..51f00fe20e4d 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(arg2, arg3, arg4, + (char __user *) arg5); + break; default: error = -EINVAL; break;

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.

Signed-off-by: Gabriel Krisman Bertazi krisman@collabora.com Acked-by: Kees Cook keescook@chromium.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org

--- Changes since v6: - Update selftests to reflect {offset,len} api change 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 --- tools/testing/selftests/Makefile | 1 + .../syscall_user_dispatch/.gitignore | 3 + .../selftests/syscall_user_dispatch/Makefile | 9 + .../selftests/syscall_user_dispatch/config | 1 + .../syscall_user_dispatch/sud_test.c | 310 ++++++++++++++++++ 5 files changed, 324 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/sud_test.c

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile index 2e20e30a6faa..e93f10386e76 100644 --- a/tools/testing/selftests/Makefile +++ b/tools/testing/selftests/Makefile @@ -57,6 +57,7 @@ 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..f539615ad5da --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/.gitignore @@ -0,0 +1,3 @@ +# SPDX-License-Identifier: GPL-2.0-only +sud_test +sud_benchmark diff --git a/tools/testing/selftests/syscall_user_dispatch/Makefile b/tools/testing/selftests/syscall_user_dispatch/Makefile new file mode 100644 index 000000000000..8e15fa42bcda --- /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 := sud_test +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..039e303e59d7 --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/config @@ -0,0 +1 @@ +CONFIG_GENERIC_ENTRY=y diff --git a/tools/testing/selftests/syscall_user_dispatch/sud_test.c b/tools/testing/selftests/syscall_user_dispatch/sud_test.c new file mode 100644 index 000000000000..6498b050ef89 --- /dev/null +++ b/tools/testing/selftests/syscall_user_dispatch/sud_test.c @@ -0,0 +1,310 @@ +// 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; + + /* offset != 0 */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0x0, 0); + EXPECT_EQ(EINVAL, errno); + + /* len != 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; + + /* Dispatcher region is bad (offset > 0 && len == 0) */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0x0, &sel); + EXPECT_EQ(EINVAL, errno); + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, -1L, 0x0, &sel); + EXPECT_EQ(EINVAL, errno); + + /* Invalid selector */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x1, (void *) -1); + ASSERT_EQ(EFAULT, errno); + + /* + * Dispatcher range overflows unsigned long + */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 1, -1L, &sel); + ASSERT_EQ(EINVAL, errno) { + TH_LOG("Should reject bad syscall range"); + } + + /* + * Allowed range overflows usigned long + */ + prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, -1L, 0x1, &sel); + ASSERT_EQ(EINVAL, errno) { + TH_LOG("Should reject bad syscall range"); + } +} + +/* + * 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 Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org

--- Changes since v7: - Change process -> thread (Florian Weimer) - Drop bogus reference to CONFIG_SYSCALL_USER_DISPATCH (me) - Document the interval as a half-open interval (me) --- .../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..0ee7491440b3 --- /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 thread can setup this mechanism on supported kernels by executing the +following prctl: + + prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [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. + +[<offset>, <offset>+<length>) delimit a 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.