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Subject[PATCH RFC] kernel: Implement selective syscall redirection through personality
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The problem I'm trying to solve is that modern Windows applications are
executing system call instructions directly from the application's code
without going through the WinAPI. This breaks Wine emulation, because
it doesn't have a chance to intercept and emulate these syscalls before
they are submitted to Linux. There are additional constraints like
performance and inability to dinamically modify the original source
code, which stop us from using regular seccomp filters or patching
userspace. Those are discussed in the previous discussion in the
mailing list.

The first approach I proposed was a new Seccomp mode to do the trapping
depending on which part of the user memory map issued the syscall. The
main problem with that approach was the use of seccomp for a non-secure
mechanism. A few other suggestions were made by the community around at
that time:

- A personality mechanism

- A quick redirection to a thunk code in userspace to do the
syscall filtering in userspace and issue the syscalls from
a well-known dispatcher

The previous discussion is archived at:
<https://lwn.net/ml/linux-kernel/20200530055953.817666-1-krisman@collabora.com/>

I experimented with both approaches and the biggest problem with the
thunk is that native Linux syscalls would also be redirected to
userspace, which added a prohibitive cost for us. In fact, for quick
syscalls like sysinfo, it almost doubled the execution time of our
benchmarks. My current understanding is that we cannot do without some
kind of kernel filtering, since it avoids the high overhead to native
Linux syscalls.

A version of the personality mechanism is what I implement in the patch
below. In order to avoid issuing personality syscalls in each boundary
transition, which we observed to be too prohibitive, it uses a
userspace-defined key to select the personality. I confess this is a
bit of an awkward interface to me, but it yields very good results.
While it adds a user memory access to every syscall when the mechanism
is enabled, the impact was tiny, even for a small syscall. More details
in the patch below.

>8

Introduce a mechanism to quickly disable/enable syscall handling for a
specific process and redirect to userspace via SIGSYS. This is useful
for processes that have different personalities depending on which part
of the application is executing, but who need to perform the personality
switch really fast, without paying the cost of a system call on each
boundary transition. This is particularly important for Windows games
running over Wine.

The proposed interface looks like this:

prctl(PR_SET_SYSCALL_DISABLE, <mode>, <dispatcher>, [fast_selector])

Dispatcher is the address of a syscall instruction that is allowed to
by-pass the blockage, such that in fast paths you don't need to disable
the trap nor check the fast selector. This is essential to return from
SIGSYS to a blocked area without triggering another SIGSYS from the
rt_sigreturn.

fast-selector is an optional pointer to an aligned int-sized userspace
memory region that has a key switch for the mechanism. The only policy
the kernel enforces on that area is that zero means SYSCALL_ALLOW while
!0 means SYSCALL_BLOCK.

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 dispatcher to issue syscalls is pretty much
irrelevant. The overhead of fast-selector goes around 40ns for a
native (untrapped) 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.

Right now, it is only supported by x86_64 and x86, but it should be
easily enabled for other architectures.

An example code using this interface can be found at:
https://gitlab.collabora.com/krisman/syscall-disable-personality

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Paul Gofman <gofmanp@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
---
arch/Kconfig | 19 +++++++
arch/x86/Kconfig | 1 +
arch/x86/entry/common.c | 7 +++
arch/x86/include/asm/thread_info.h | 4 +-
fs/exec.c | 4 ++
include/linux/sched.h | 5 ++
include/linux/syscall_disable.h | 29 ++++++++++
include/uapi/asm-generic/siginfo.h | 1 +
include/uapi/linux/prctl.h | 4 ++
kernel/Makefile | 1 +
kernel/fork.c | 4 ++
kernel/sys.c | 7 +++
kernel/syscall_disable.c | 85 ++++++++++++++++++++++++++++++
13 files changed, 170 insertions(+), 1 deletion(-)
create mode 100644 include/linux/syscall_disable.h
create mode 100644 kernel/syscall_disable.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..bbbfbd4b9f86 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -465,6 +465,25 @@ config SECCOMP_FILTER

See Documentation/userspace-api/seccomp_filter.rst for details.

+config HAVE_ARCH_FAST_SYSCALL_DISABLE
+ bool
+ help
+ An arch should select this symbol if it provides all of these things:
+ - TIF_FAST_SYSCALL_DISABLE
+ - syscall_get_arch
+ - syscall_rollback
+ - SIGSYS siginfo_t support
+
+config FAST_SYSCALL_DISABLE
+ bool "Support rejecting syscalls not coming from a dispatcher"
+ depends on HAVE_ARCH_FAST_SYSCALL_DISABLE
+ help
+ Enable tasks to ask the kernel to reject syscalls not
+ issued from a predefined dispatcher depending on a userspace
+ key.
+
+ This option is useful to optimize games running over Wine.
+
config HAVE_ARCH_STACKLEAK
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 883da0abf779..36f711570d19 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -149,6 +149,7 @@ config X86
select HAVE_ARCH_COMPAT_MMAP_BASES if MMU && COMPAT
select HAVE_ARCH_PREL32_RELOCATIONS
select HAVE_ARCH_SECCOMP_FILTER
+ select HAVE_ARCH_FAST_SYSCALL_DISABLE
select HAVE_ARCH_THREAD_STRUCT_WHITELIST
select HAVE_ARCH_STACKLEAK
select HAVE_ARCH_TRACEHOOK
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index bd3f14175193..26b3afbeea37 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -138,6 +138,13 @@ static long syscall_trace_enter(struct pt_regs *regs)
return -1L;
}

+#ifdef CONFIG_FAST_SYSCALL_DISABLE
+ if (work & _TIF_FAST_SYSCALL_DISABLE) {
+ if (do_fast_syscall_disable(regs->orig_ax, regs))
+ return -1L;
+ }
+#endif
+
#ifdef CONFIG_SECCOMP
/*
* Do seccomp after ptrace, to catch any tracer changes.
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 8de8ceccb8bc..be72d14fc570 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -93,6 +93,7 @@ struct thread_info {
#define TIF_NOTSC 16 /* TSC is not accessible in userland */
#define TIF_IA32 17 /* IA32 compatibility process */
#define TIF_SLD 18 /* Restore split lock detection on context switch */
+#define TIF_FAST_SYSCALL_DISABLE 19 /* fast syscall disable active */
#define TIF_MEMDIE 20 /* is terminating due to OOM killer */
#define TIF_POLLING_NRFLAG 21 /* idle is polling for TIF_NEED_RESCHED */
#define TIF_IO_BITMAP 22 /* uses I/O bitmap */
@@ -123,6 +124,7 @@ struct thread_info {
#define _TIF_NOTSC (1 << TIF_NOTSC)
#define _TIF_IA32 (1 << TIF_IA32)
#define _TIF_SLD (1 << TIF_SLD)
+#define _TIF_FAST_SYSCALL_DISABLE (1 << TIF_FAST_SYSCALL_DISABLE)
#define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG)
#define _TIF_IO_BITMAP (1 << TIF_IO_BITMAP)
#define _TIF_FORCED_TF (1 << TIF_FORCED_TF)
@@ -136,7 +138,7 @@ struct thread_info {
/* Work to do before invoking the actual syscall. */
#define _TIF_WORK_SYSCALL_ENTRY \
(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU | _TIF_SYSCALL_AUDIT | \
- _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT)
+ _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | _TIF_FAST_SYSCALL_DISABLE)

/* flags to check in __switch_to() */
#define _TIF_WORK_CTXSW_BASE \
diff --git a/fs/exec.c b/fs/exec.c
index e6e8a9a70327..3c40b00aa19f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1502,6 +1502,10 @@ void setup_new_exec(struct linux_binprm * bprm)

arch_setup_new_exec();

+#ifdef CONFIG_FAST_SYSCALL_DISABLE
+ clear_tsk_thread_flag(me, TIF_FAST_SYSCALL_DISABLE);
+#endif
+
/* Set the new mm task size. We have to do that late because it may
* depend on TIF_32BIT which is only updated in flush_thread() on
* some architectures like powerpc
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 692e327d7455..f502b7fc2bfd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -32,6 +32,7 @@
#include <linux/posix-timers.h>
#include <linux/rseq.h>
#include <linux/kcsan.h>
+#include <linux/syscall_disable.h>

/* task_struct member predeclarations (sorted alphabetically): */
struct audit_context;
@@ -953,6 +954,10 @@ struct task_struct {
#endif
struct seccomp seccomp;

+#ifdef CONFIG_FAST_SYSCALL_DISABLE
+ struct fast_syscall_disable syscall_disable;
+#endif
+
/* Thread group tracking: */
u64 parent_exec_id;
u64 self_exec_id;
diff --git a/include/linux/syscall_disable.h b/include/linux/syscall_disable.h
new file mode 100644
index 000000000000..96e52f50eee5
--- /dev/null
+++ b/include/linux/syscall_disable.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _SYSCALL_DISABLE_H
+#define _SYSCALL_DISABLE_H
+
+#ifdef CONFIG_FAST_SYSCALL_DISABLE
+struct fast_syscall_disable {
+ int __user *fastlock;
+ unsigned long __user dispatcher;
+};
+
+int do_fast_syscall_disable(int nr_syscall, struct pt_regs *regs);
+int set_syscall_disable(int mode, unsigned long __user dispatcher,
+ int __user *fastlock);
+
+#else
+static inline int set_syscall_disable(int mode, unsigned long __user dispatcher,
+ int __user *fastlock)
+{
+ return -EINVAL;
+}
+
+static inline int do_fast_syscall_disable(int nr_syscall, struct pt_regs *regs)
+{
+ WARN_ON(1);
+ return 0;
+}
+#endif /* CONFIG_FAST_SYSCALL_DISABLE */
+
+#endif
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index cb3d6c267181..09d4ef45a341 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -285,6 +285,7 @@ typedef struct siginfo {
*/
#define SYS_SECCOMP 1 /* seccomp triggered */
#define NSIGSYS 1
+#define SYS_DISABLED 2

/*
* SIGEMT si_codes
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..383773269ade 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,8 @@ struct prctl_mm_map {
#define PR_SET_IO_FLUSHER 57
#define PR_GET_IO_FLUSHER 58

+#define PR_SET_SYSCALL_DISABLE 59
+# define PR_SYSCALL_ALLOW 0
+# define PR_SYSCALL_BLOCK 1
+
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/Makefile b/kernel/Makefile
index f3218bc5ec69..e0449ed68fd5 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -92,6 +92,7 @@ obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
obj-$(CONFIG_LOCKUP_DETECTOR) += watchdog.o
obj-$(CONFIG_HARDLOCKUP_DETECTOR_PERF) += watchdog_hld.o
obj-$(CONFIG_SECCOMP) += seccomp.o
+obj-$(CONFIG_FAST_SYSCALL_DISABLE) += syscall_disable.o
obj-$(CONFIG_RELAY) += relay.o
obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
diff --git a/kernel/fork.c b/kernel/fork.c
index 142b23645d82..529f0b5e02d3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -922,6 +922,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
clear_tsk_need_resched(tsk);
set_task_stack_end_magic(tsk);

+#ifdef CONFIG_FAST_SYSCALL_DISABLE
+ clear_tsk_thread_flag(tsk, TIF_FAST_SYSCALL_DISABLE);
+#endif
+
#ifdef CONFIG_STACKPROTECTOR
tsk->stack_canary = get_random_canary();
#endif
diff --git a/kernel/sys.c b/kernel/sys.c
index 00a96746e28a..2205df99ba67 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_disable.h>

#include <linux/compat.h>
#include <linux/syscalls.h>
@@ -2527,6 +2528,12 @@ 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_DISABLE:
+ if (arg5)
+ return -EINVAL;
+ error = set_syscall_disable((int) arg2, (unsigned long __user) arg3,
+ (int __user *) arg4);
+ break;
default:
error = -EINVAL;
break;
diff --git a/kernel/syscall_disable.c b/kernel/syscall_disable.c
new file mode 100644
index 000000000000..4b0a1f916df3
--- /dev/null
+++ b/kernel/syscall_disable.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/sched.h>
+#include <linux/prctl.h>
+#include <linux/syscall_disable.h>
+#include <linux/uaccess.h>
+#include <linux/signal.h>
+#include <linux/processor.h>
+
+#include <asm/syscall.h>
+
+#include <linux/sched/signal.h>
+#include <linux/sched/task_stack.h>
+
+static void trigger_sigsys(int nr_syscall)
+{
+ struct kernel_siginfo info;
+
+ clear_siginfo(&info);
+ info.si_signo = SIGSYS;
+ info.si_code = SYS_DISABLED;
+ info.si_call_addr = (void __user *)KSTK_EIP(current);
+ info.si_errno = 0;
+ info.si_arch = syscall_get_arch(current);
+ info.si_syscall = nr_syscall;
+
+ force_sig_info(&info);
+}
+
+int do_fast_syscall_disable(int nr_syscall, struct pt_regs *regs)
+{
+ int state;
+
+ if (current->syscall_disable.dispatcher == instruction_pointer(regs))
+ return 0;
+
+ if (current->syscall_disable.fastlock) {
+ if (__get_user(state, current->syscall_disable.fastlock))
+ do_exit(SIGSEGV);
+
+ if (!state)
+ return 0;
+ }
+
+ syscall_rollback(current, regs);
+ trigger_sigsys(nr_syscall);
+
+ return 1;
+}
+
+int set_syscall_disable(int mode, unsigned long __user dispatcher,
+ int __user *fastlock)
+{
+ switch (mode) {
+ case PR_SYSCALL_ALLOW:
+ if (dispatcher || fastlock)
+ return -EINVAL;
+ break;
+ case PR_SYSCALL_BLOCK:
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ if (fastlock) {
+ if (!access_ok(fastlock, sizeof(int)))
+ return -EFAULT;
+ }
+
+ spin_lock_irq(&current->sighand->siglock);
+
+ current->syscall_disable.fastlock = fastlock;
+ current->syscall_disable.dispatcher = dispatcher;
+
+ /* make sure fsb_address is committed before setting the flag. */
+ smp_mb__before_atomic();
+
+ if (mode == PR_SYSCALL_BLOCK)
+ set_tsk_thread_flag(current, TIF_FAST_SYSCALL_DISABLE);
+ else
+ clear_tsk_thread_flag(current, TIF_FAST_SYSCALL_DISABLE);
+
+ spin_unlock_irq(&current->sighand->siglock);
+
+ return 0;
+}
--
2.27.0
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