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 af14a567b493..6e022c5de020 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 a91003e28eaa..9cdf827151c4 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> @@ -1418,6 +1419,13 @@ 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 3511c98a7849..47ce09589072 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>

/* task_struct member predeclarations (sorted alphabetically): */ struct audit_context; @@ -963,6 +964,7 @@ 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 = &current->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 ab6c409b1159..2786dca862b9 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;

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 9018f45d631d..34ab8579e22f 100644 --- a/tools/testing/selftests/Makefile +++ b/tools/testing/selftests/Makefile @@ -56,6 +56,7 @@ 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.