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

[Kees Cook]

On Fri, Jan 16, 2015 at 11:57 AM, Kees Cook <keescook at chromium.org> wrote:
> 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...)

So, some further testing:
- native arm64 presents "poll" again even to seccomp when
restart_syscall is triggered (both via regs[8] and
NT_ARM_SYSTEM_CALL).
- compat mode on arm64 _does_ show syscall_restart (via ARM_r7).

Which of these behaviors is intentional? :)

-Kees

-- 
Kees Cook
Chrome OS Security