[v4,1/2] kernel: Implement selective syscall userspace redirection
diff mbox series

Message ID 20200716193141.4068476-2-krisman@collabora.com
State New
Headers show
Series
  • Syscall User Redirection
Related show

Commit Message

Gabriel Krisman Bertazi July 16, 2020, 7:31 p.m. UTC
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.

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

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
---
 arch/Kconfig                          | 20 ++++++
 arch/x86/Kconfig                      |  1 +
 arch/x86/entry/common.c               |  5 ++
 arch/x86/include/asm/thread_info.h    |  4 +-
 arch/x86/kernel/signal_compat.c       |  2 +-
 fs/exec.c                             |  2 +
 include/linux/sched.h                 |  3 +
 include/linux/syscall_user_dispatch.h | 50 +++++++++++++++
 include/uapi/asm-generic/siginfo.h    |  3 +-
 include/uapi/linux/prctl.h            |  5 ++
 kernel/Makefile                       |  1 +
 kernel/fork.c                         |  1 +
 kernel/sys.c                          |  5 ++
 kernel/syscall_user_dispatch.c        | 92 +++++++++++++++++++++++++++
 14 files changed, 191 insertions(+), 3 deletions(-)
 create mode 100644 include/linux/syscall_user_dispatch.h
 create mode 100644 kernel/syscall_user_dispatch.c

Comments

Matthew Wilcox July 16, 2020, 9:06 p.m. UTC | #1
On Thu, Jul 16, 2020 at 03:31:40PM -0400, Gabriel Krisman Bertazi wrote:
> 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.

Disabled on exec.  Disabled in the child on clone/fork (and vfork, I
think).

That means we don't need to worry about it interacting badly with
a setuid program, right?
Kees Cook July 16, 2020, 9:26 p.m. UTC | #2
On Thu, Jul 16, 2020 at 10:06:01PM +0100, Matthew Wilcox wrote:
> On Thu, Jul 16, 2020 at 03:31:40PM -0400, Gabriel Krisman Bertazi wrote:
> > 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.
> 
> Disabled on exec.  Disabled in the child on clone/fork (and vfork, I
> think).
> 
> That means we don't need to worry about it interacting badly with
> a setuid program, right?

Right, that's the intention.
Andy Lutomirski July 17, 2020, 12:20 a.m. UTC | #3
On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>

This is quite nice.  I have a few comments, though:

You mentioned rt_sigreturn().  Should this automatically exempt the
kernel-provided signal restorer on architectures (e.g. x86_32) that
provide one?

The amount of syscall entry wiring that arches need to do is IMO
already a bit out of hand.  Should we instead rename TIF_SECCOMP to
TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
seccomp and this new thing?

> +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 (likely(sd->selector)) {
> +               if (unlikely(__get_user(state, sd->selector)))
> +                       do_exit(SIGSEGV);
> +
> +               if (likely(state == 0))
> +                       return 0;
> +
> +               if (state != 1)
> +                       do_exit(SIGSEGV);

This seems a bit extreme and hard to debug if it ever happens.
Gabriel Krisman Bertazi July 17, 2020, 2:15 a.m. UTC | #4
Andy Lutomirski <luto@kernel.org> writes:

> On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
> <krisman@collabora.com> wrote:
>>
>
> This is quite nice.  I have a few comments, though:
>
> You mentioned rt_sigreturn().  Should this automatically exempt the
> kernel-provided signal restorer on architectures (e.g. x86_32) that
> provide one?

That seems reasonable.  Not sure how easy it is to do it, though.

> The amount of syscall entry wiring that arches need to do is IMO
> already a bit out of hand.  Should we instead rename TIF_SECCOMP to
> TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
> seccomp and this new thing?

Considering the previous suggestion from Kees to hide it inside the
tracehook and Thomas rework of this path, I'm not sure what is the best
solution here, but some rework of these flags is due.  Thomas suggested
expanding these flags to 64 bits and having some arch specific and
arch-agnostic flags.  With the storage expansion and arch-agnostic flags,
would this still be desirable?

>> +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 (likely(sd->selector)) {
>> +               if (unlikely(__get_user(state, sd->selector)))
>> +                       do_exit(SIGSEGV);
>> +
>> +               if (likely(state == 0))
>> +                       return 0;
>> +
>> +               if (state != 1)
>> +                       do_exit(SIGSEGV);
>
> This seems a bit extreme and hard to debug if it ever happens.

Makes sense, but I don't see a better way to return the error here.
Maybe a SIGSYS with a different si_errno?  Alternatively, we could
revert to the previous behavior of allowing syscalls on state != 0, that
existed in v1.  What do you think?
Andy Lutomirski July 17, 2020, 4:48 a.m. UTC | #5
On Thu, Jul 16, 2020 at 7:15 PM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>
> Andy Lutomirski <luto@kernel.org> writes:
>
> > On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
> > <krisman@collabora.com> wrote:
> >>
> >
> > This is quite nice.  I have a few comments, though:
> >
> > You mentioned rt_sigreturn().  Should this automatically exempt the
> > kernel-provided signal restorer on architectures (e.g. x86_32) that
> > provide one?
>
> That seems reasonable.  Not sure how easy it is to do it, though.

For better or for worse, it's currently straightforward because the code is:

__kernel_sigreturn:
.LSTART_sigreturn:
        popl %eax               /* XXX does this mean it needs unwind info? */
        movl $__NR_sigreturn, %eax
        SYSCALL_ENTER_KERNEL

and SYSCALL_ENTER_KERNEL is hardwired as int $0x80.  (The latter is
probably my fault, for better or for worse.)  So this would change to:

__vdso32_sigreturn_syscall:
  SYSCALL_ENTER_KERNEL

and vdso2c would wire up __vdso32_sigreturn_syscall.  Then there would
be something like:

bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs);

and that would be that.  Does anyone have an opinion as to whether
this is a good idea?  Modern glibc shouldn't be using this mechanism,
I think, but I won't swear to it.

>
> > The amount of syscall entry wiring that arches need to do is IMO
> > already a bit out of hand.  Should we instead rename TIF_SECCOMP to
> > TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
> > seccomp and this new thing?
>
> Considering the previous suggestion from Kees to hide it inside the
> tracehook and Thomas rework of this path, I'm not sure what is the best
> solution here, but some rework of these flags is due.  Thomas suggested
> expanding these flags to 64 bits and having some arch specific and
> arch-agnostic flags.  With the storage expansion and arch-agnostic flags,
> would this still be desirable?

I think it would be desirable to consolidate this to avoid having
multiple arches need to separately wire up all of these mechanisms.
I'm not sure that the initial upstream implementation needs this, but
it might be nice to support this out of the box on all arches with
seccomp support.

>
> >> +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 (likely(sd->selector)) {
> >> +               if (unlikely(__get_user(state, sd->selector)))
> >> +                       do_exit(SIGSEGV);
> >> +
> >> +               if (likely(state == 0))
> >> +                       return 0;
> >> +
> >> +               if (state != 1)
> >> +                       do_exit(SIGSEGV);
> >
> > This seems a bit extreme and hard to debug if it ever happens.
>
> Makes sense, but I don't see a better way to return the error here.
> Maybe a SIGSYS with a different si_errno?  Alternatively, we could
> revert to the previous behavior of allowing syscalls on state != 0, that
> existed in v1.  What do you think?
>

I don't have a strong opinion.  SIGSYS with different si_errno is
probably reasonable.

--Andy
Thomas Gleixner July 20, 2020, 9:23 a.m. UTC | #6
Andy Lutomirski <luto@kernel.org> writes:
> On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
> <krisman@collabora.com> wrote:
> The amount of syscall entry wiring that arches need to do is IMO
> already a bit out of hand.  Should we instead rename TIF_SECCOMP to
> TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
> seccomp and this new thing?

The right way to go is to consolidate all the stupidly different
entry/exit work handling implementations and have exactly one in generic
code, i.e. what I posted a few days ago.

Then we can make new features only available in the generic version by
hiding the new functionality in the core code and not exposing the
functions to architecture implementations.

Making it easy for architectures to keep their own variant forever just
proliferates the mess we have right now.

Thanks,

        tglx
Will Deacon July 20, 2020, 9:44 a.m. UTC | #7
On Mon, Jul 20, 2020 at 11:23:13AM +0200, Thomas Gleixner wrote:
> Andy Lutomirski <luto@kernel.org> writes:
> > On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
> > <krisman@collabora.com> wrote:
> > The amount of syscall entry wiring that arches need to do is IMO
> > already a bit out of hand.  Should we instead rename TIF_SECCOMP to
> > TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
> > seccomp and this new thing?
> 
> The right way to go is to consolidate all the stupidly different
> entry/exit work handling implementations and have exactly one in generic
> code, i.e. what I posted a few days ago.
> 
> Then we can make new features only available in the generic version by
> hiding the new functionality in the core code and not exposing the
> functions to architecture implementations.
> 
> Making it easy for architectures to keep their own variant forever just
> proliferates the mess we have right now.

Couldn't agree more. We recently added PTRACE_SYSEMU to arm64 and I deeply
regret doing that now that yet another way to rewrite the syscall number
has come along. I only just untangled some of the mess in our entry code
for that, so I can't say I'm looking forward to opening it right back up
to support this new feature. Much better to do it in the core code instead.

Will
Thomas Gleixner July 20, 2020, 10:08 a.m. UTC | #8
Gabriel,

Gabriel Krisman Bertazi <krisman@collabora.com> writes:
> 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.

Why isn't rt_sigreturn() exempt from that redirection in the first place?

> ---
>  arch/Kconfig                          | 20 ++++++
>  arch/x86/Kconfig                      |  1 +
>  arch/x86/entry/common.c               |  5 ++
>  arch/x86/include/asm/thread_info.h    |  4 +-
>  arch/x86/kernel/signal_compat.c       |  2 +-
>  fs/exec.c                             |  2 +
>  include/linux/sched.h                 |  3 +
>  include/linux/syscall_user_dispatch.h | 50 +++++++++++++++
>  include/uapi/asm-generic/siginfo.h    |  3 +-
>  include/uapi/linux/prctl.h            |  5 ++
>  kernel/Makefile                       |  1 +
>  kernel/fork.c                         |  1 +
>  kernel/sys.c                          |  5 ++
>  kernel/syscall_user_dispatch.c        | 92 +++++++++++++++++++++++++++

A big combo patch is not how we do that. Please split it up into the
core part and a patch enabling it for a particular architexture.

As I said in my reply to Andy, this wants to go on top of the generic
entry/exit work stuff:

  https://lore.kernel.org/r/20200716182208.180916541@linutronix.de

and then syscall_user_dispatch.c ends up in kernel/entry/ and the
dispatching function is not exposed outside of that directory.

I'm going to post a new version later today. Will cc you.

> --- 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_SYSCALL_USER_DISPATCH 19	/* Redirect syscall for userspace handling */

There are two other things out there which compete about the last TIF
bits on x86, so we need to clean that up first.

> +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 (likely(sd->selector)) {
> +		if (unlikely(__get_user(state, sd->selector)))

__get_user() mandates an explicit access_ok() which happened in the
prctl(). So this wants a comment why there is none right here.

> +			do_exit(SIGSEGV);
> +
> +		if (likely(state == 0))
> +			return 0;
> +
> +		if (state != 1)
> +			do_exit(SIGSEGV);

If that happens its going to be quite interesting to debug.

Also please use proper defines which are exposed to user space instead
of 0/1.

> +	}
> +
> +	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, 1))

  sizeof(*selector)

Thanks,

        tglx
Gabriel Krisman Bertazi July 20, 2020, 1:46 p.m. UTC | #9
Hi Thomas,

Thanks for the valuable feedback!

Thomas Gleixner <tglx@linutronix.de> writes:
> Gabriel Krisman Bertazi <krisman@collabora.com> writes:
>> 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.
>
> Why isn't rt_sigreturn() exempt from that redirection in the first
> place?

This was actually a design decision for me.

The main use case I'm considering is emulation of applications written
for other OSs (games over wine), which means this dispatcher code is
exposed to applications built against different ABIs, who trigger
syscalls with bogus parameters (from a linux perspective)

In this emulation scenario, I cannot really trust the syscall number
means rt_sigreturn, so I try to only base the dispatcher decision on the
memory region and selector variable.

I think the best we can do is what Andy said: to exempt rt_sigreturn
when it comes from the vdso, for architectures that do it that way.

>
>> ---
>>  arch/Kconfig                          | 20 ++++++
>>  arch/x86/Kconfig                      |  1 +
>>  arch/x86/entry/common.c               |  5 ++
>>  arch/x86/include/asm/thread_info.h    |  4 +-
>>  arch/x86/kernel/signal_compat.c       |  2 +-
>>  fs/exec.c                             |  2 +
>>  include/linux/sched.h                 |  3 +
>>  include/linux/syscall_user_dispatch.h | 50 +++++++++++++++
>>  include/uapi/asm-generic/siginfo.h    |  3 +-
>>  include/uapi/linux/prctl.h            |  5 ++
>>  kernel/Makefile                       |  1 +
>>  kernel/fork.c                         |  1 +
>>  kernel/sys.c                          |  5 ++
>>  kernel/syscall_user_dispatch.c        | 92 +++++++++++++++++++++++++++
>
> A big combo patch is not how we do that. Please split it up into the
> core part and a patch enabling it for a particular architexture.
>
> As I said in my reply to Andy, this wants to go on top of the generic
> entry/exit work stuff:
>
>   https://lore.kernel.org/r/20200716182208.180916541@linutronix.de
>
> and then syscall_user_dispatch.c ends up in kernel/entry/ and the
> dispatching function is not exposed outside of that directory.
>
> I'm going to post a new version later today. Will cc you.

Thanks. Will do!
Mark Rutland July 21, 2020, 12:06 p.m. UTC | #10
On Thu, Jul 16, 2020 at 09:48:50PM -0700, Andy Lutomirski wrote:
> On Thu, Jul 16, 2020 at 7:15 PM Gabriel Krisman Bertazi
> <krisman@collabora.com> wrote:
> >
> > Andy Lutomirski <luto@kernel.org> writes:
> >
> > > On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
> > > <krisman@collabora.com> wrote:
> > >>
> > >
> > > This is quite nice.  I have a few comments, though:
> > >
> > > You mentioned rt_sigreturn().  Should this automatically exempt the
> > > kernel-provided signal restorer on architectures (e.g. x86_32) that
> > > provide one?
> >
> > That seems reasonable.  Not sure how easy it is to do it, though.
> 
> For better or for worse, it's currently straightforward because the code is:
> 
> __kernel_sigreturn:
> .LSTART_sigreturn:
>         popl %eax               /* XXX does this mean it needs unwind info? */
>         movl $__NR_sigreturn, %eax
>         SYSCALL_ENTER_KERNEL
> 
> and SYSCALL_ENTER_KERNEL is hardwired as int $0x80.  (The latter is
> probably my fault, for better or for worse.)  So this would change to:
> 
> __vdso32_sigreturn_syscall:
>   SYSCALL_ENTER_KERNEL
> 
> and vdso2c would wire up __vdso32_sigreturn_syscall.  Then there would
> be something like:
> 
> bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs);
> 
> and that would be that.  Does anyone have an opinion as to whether
> this is a good idea?  Modern glibc shouldn't be using this mechanism,
> I think, but I won't swear to it.

On arm64 sigreturn is always through the vdso, so IIUC we'd certainly
need something like this. Otherwise it'd be the user's responsibility to
register the vdso sigtramp range when making the prctl, and flip the
selector in each signal handler, which sounds both painful and fragile.

Mark.

Patch
diff mbox series

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..0ebd971d0d8f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -465,6 +465,26 @@  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_USER_DISPATCH
+	  - syscall_get_arch
+	  - syscall_rollback
+	  - syscall_get_nr
+	  - SIGSYS siginfo_t support
+
+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/arch/x86/Kconfig b/arch/x86/Kconfig
index 883da0abf779..466a3a9c0708 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_SYSCALL_USER_DISPATCH
 	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..6c1360a7f260 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -138,6 +138,11 @@  static long syscall_trace_enter(struct pt_regs *regs)
 			return -1L;
 	}
 
+	if (work & _TIF_SYSCALL_USER_DISPATCH) {
+		if (do_syscall_user_dispatch(regs))
+			return -1L;
+	}
+
 #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..b26a9f2f0491 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_SYSCALL_USER_DISPATCH 19	/* Redirect syscall for userspace handling */
 #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_SYSCALL_USER_DISPATCH (1 << TIF_SYSCALL_USER_DISPATCH)
 #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_SYSCALL_USER_DISPATCH)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW_BASE					\
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/fs/exec.c b/fs/exec.c
index e6e8a9a70327..849f618ed790 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1386,6 +1386,8 @@  int begin_new_exec(struct linux_binprm * bprm)
 	flush_thread();
 	me->personality &= ~bprm->per_clear;
 
+	clear_tsk_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 692e327d7455..407b868146e1 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_user_dispatch.h>
 
 /* task_struct member predeclarations (sorted alphabetically): */
 struct audit_context;
@@ -953,6 +954,8 @@  struct task_struct {
 #endif
 	struct seccomp			seccomp;
 
+	struct syscall_user_dispatch	syscall_dispatch;
+
 	/* Thread group tracking: */
 	u64				parent_exec_id;
 	u64				self_exec_id;
diff --git a/include/linux/syscall_user_dispatch.h b/include/linux/syscall_user_dispatch.h
new file mode 100644
index 000000000000..a49e2de93705
--- /dev/null
+++ b/include/linux/syscall_user_dispatch.h
@@ -0,0 +1,50 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2020 Collabora Ltd.
+ */
+#ifndef _SYSCALL_USER_DISPATCH_H
+#define _SYSCALL_USER_DISPATCH_H
+
+struct task_struct;
+static void clear_tsk_thread_flag(struct task_struct *tsk, int flag);
+
+#ifdef CONFIG_SYSCALL_USER_DISPATCH
+struct syscall_user_dispatch {
+	char __user *selector;
+	unsigned long dispatcher_start;
+	unsigned long dispatcher_end;
+};
+
+int do_syscall_user_dispatch(struct pt_regs *regs);
+int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start,
+			      unsigned long dispatcher_end,
+			      char __user *selector);
+
+static inline void clear_tsk_syscall_user_dispatch(struct task_struct *tsk)
+{
+	clear_tsk_thread_flag(tsk, TIF_SYSCALL_USER_DISPATCH);
+}
+
+#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;
+}
+
+static inline int do_syscall_user_dispatch(struct pt_regs *regs)
+{
+	WARN_ON_ONCE(1);
+	return 0;
+}
+
+static inline void clear_tsk_syscall_user_dispatch(struct task_struct *tsk)
+{
+}
+
+#endif /* CONFIG_SYSCALL_USER_DISPATCH */
+
+#endif /* _SYSCALL_USER_DISPATCH_H */
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
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/Makefile b/kernel/Makefile
index f3218bc5ec69..158b8c61592f 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-$(CONFIG_SYSCALL_USER_DISPATCH) += syscall_user_dispatch.o
 obj-$(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..c6b64a849fec 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -921,6 +921,7 @@  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_tsk_syscall_user_dispatch(tsk);
 
 #ifdef CONFIG_STACKPROTECTOR
 	tsk->stack_canary = get_random_canary();
diff --git a/kernel/sys.c b/kernel/sys.c
index 00a96746e28a..d85880873c92 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>
@@ -2527,6 +2528,10 @@  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;
diff --git a/kernel/syscall_user_dispatch.c b/kernel/syscall_user_dispatch.c
new file mode 100644
index 000000000000..39ee29c2b91f
--- /dev/null
+++ b/kernel/syscall_user_dispatch.c
@@ -0,0 +1,92 @@ 
+// 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 <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 (likely(sd->selector)) {
+		if (unlikely(__get_user(state, sd->selector)))
+			do_exit(SIGSEGV);
+
+		if (likely(state == 0))
+			return 0;
+
+		if (state != 1)
+			do_exit(SIGSEGV);
+	}
+
+	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, 1))
+			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_thread_flag(current, TIF_SYSCALL_USER_DISPATCH);
+	else
+		clear_tsk_thread_flag(current, TIF_SYSCALL_USER_DISPATCH);
+
+	return 0;
+}