[PATCH 1/2] ARM: entry-common: fix forgotten set of thread_info->syscall

[Kees Cook]

On Fri, Jan 16, 2015 at 8:17 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Sat, Jan 17, 2015 at 01:08:11AM +0900, Roman Peniaev wrote:
>> On Sat, Jan 17, 2015 at 12:59 AM, Russell King - ARM Linux
>> <linux at arm.linux.org.uk> wrote:
>> > On Sat, Jan 17, 2015 at 12:57:02AM +0900, Roman Peniaev wrote:
>> >> On Fri, Jan 16, 2015 at 7:54 AM, Kees Cook <keescook at chromium.org> wrote:
>> >> > One interesting thing I noticed (which is unchanged by this series),
>> >> > but pulling ARM_r7 during the seccomp ptrace event shows __NR_poll,
>> >> > not __NR_restart_syscall, even though it was a __NR_restart_syscall
>> >> > trap from seccomp. Is there a better place to see the actual syscall?
>> >>
>> >> As I understand we do not push new r7 to the stack, and ptrace uses the
>> >> old value.
>> >
>> > And why should we push r7 to the stack?  ptrace should be using the
>> > recorded system call number, rather than poking about on the stack
>> > itself.
>>
>> Probably we should not, but the behaviour comparing arm to x86 is different.
>
> We definitely should not, because changing the stacked value changes the
> value in r7 after the syscall has returned.  We have guaranteed that the
> value will be preserved across syscalls for years, so we really should
> not be changing that.

Yeah, we can't mess with the registers. I was just asking for
clarification on how this is visible to userspace.

>
>> Also there is no any way from userspace to figure out what syscall was
>> restarted, if you do not trace each syscall enter and exit from the
>> very beginning.
>
> Thinking about ptrace, that's been true for years.
>
> It really depends whether you consider the restart syscall a userspace
> thing or a kernelspace thing.  When you consider that the vast majority
> of syscall restarts are done internally in the kernel, and we just
> re-issue the syscall, it immediately brings up the question "why is
> the restart block method different?" and "should the restart block
> method be visible to userspace?"
>
> IMHO, it is prudent not to expose kernel internals to userspace unless
> there is a real reason to, otherwise they become part of the userspace
> API.

I couldn't agree more, but restart_syscall is already visible to
userspace: it can be called directly, for example. And it's visible to
tracers.

Unfortunately, the difference here is the visibility during trace
trap. On x86, it's exposed but on ARM, there's no way (that I can
find) to query the "true" syscall, even though the true syscall is
what triggers the tracer. The syscall number isn't provided by any
element of the ptrace event system, nor through siginfo, and must be
examined on a per-arch basis from registers.

Seccomp does, however, provide a mechanism to pass arbitrary event
data on a TRACE event, so poll vs restart_syscall can be distinguished
that way.

It seems even strace doesn't know how to find this information. For example:

x86:
poll([{fd=3, events=POLLIN}], 1, 4294967295
) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
--- SIGSTOP {si_signo=SIGSTOP, si_code=SI_USER, si_pid=994, si_uid=1000} ---
--- stopped by SIGSTOP ---
--- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=994, si_uid=1000} ---
restart_syscall(<... resuming interrupted call ...>

ARM:
poll([{fd=3, events=POLLIN}], 1, -1
)    = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
--- SIGSTOP {si_signo=SIGSTOP, si_code=SI_USER, si_pid=20563, si_uid=0} ---
--- stopped by SIGSTOP ---
--- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=20563, si_uid=0} ---
poll([{fd=3, events=POLLIN}], 1, -1

Would it make sense to add REGSET_SYSTEM_CALL to ARM? (Though this
begs the question, "Is restart_syscall visible during a trace on
arm64?", which I'll have to go check...)

-Kees

-- 
Kees Cook
Chrome OS Security