[PATCH] arm: ptrace: fix syscall modification under PTRACE_O_TRACESECCOMP

AKASHI Takahiro takahiro.akashi at linaro.org
Thu Jul 3 00:43:07 PDT 2014

Hi Will,

On 06/24/2014 05:54 PM, Will Deacon wrote:
> On Mon, Jun 23, 2014 at 08:46:52PM +0100, Kees Cook wrote:
>> On Mon, Jun 23, 2014 at 1:46 AM, Will Deacon <will.deacon at arm.com> wrote:
>>> On Fri, Jun 20, 2014 at 07:10:46PM +0100, Kees Cook wrote:
>>>> On Fri, Jun 20, 2014 at 10:36 AM, Kees Cook <keescook at chromium.org> wrote:
>>>>> On Fri, Jun 20, 2014 at 10:23 AM, Will Deacon <will.deacon at arm.com> wrote:
>>>>>> Right, gotcha. Thanks for the explanation. I was confused, because
>>>>>> tracehook_report_syscall does the right thing (returns
>>>>>> current_thread_info()->syscall), but if we don't have TIF_SYSCALL_TRACE set,
>>>>>> then updates during the secure_computing callback will be ignored.re
>>>>>> However, my fix to this is significantly smaller than your patch, so I fear
>>>>>> I'm still missing something.
>>>>> Oh, yes, that's much smaller. Nice! I will test this and report back.
>>>> Yup, I can confirm this works. Thanks!
>>>> Tested-by: Kees Cook <keescook at chromium.org>
>>> Thanks, Kees. I'll post a patch shortly. I'll try and remember to keep an
>>> eye out for this when seccomp lands for arm64 too.
>> Great, thanks!
>> What's the state of seccomp on arm64? I saw a series back in March,
>> but nothing since then? It looked complete, but I haven't set up a
>> test environment yet to verify.
> I think Akashi was going to repost `real soon now' so we can include them
> for 3.17. He missed the merge window last time around.

I took a quick look at the current implementation of ptrace.
ptrace(PTRACE_GETREGSET/SETREGSET), eventually gpr_get/set(), handles only 'struct user_pt_regs',
and we have no way to modify orig_x0 nor syscallno in 'struct pt_regs' directly.
So it seems to me that we can't change a system call by ptrace().
Do I misunderstand anything?

-Takahiro AKASHI

> Will
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