@@ -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
@@ -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;
@@ -965,6 +966,7 @@ struct task_struct {
unsigned int sessionid;
#endif
struct seccomp seccomp;
+ struct syscall_user_dispatch syscall_dispatch;
/* Thread group tracking: */
u64 parent_exec_id;
new file mode 100644
@@ -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 */
@@ -41,6 +41,7 @@ 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)
@@ -48,6 +49,7 @@ 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)
#include <asm/thread_info.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 */
@@ -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
new file mode 100644
@@ -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
new file mode 100644
@@ -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 = ¤t->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;
+}
@@ -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();
@@ -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;
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>+len] 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> --- 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