Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

Kees Cook
2014-09-03T18:34:44Z

On Tue, Aug 26, 2014 at 10:32 PM, AKASHI Takahiro
<takahiro.akashi@linaro.org> wrote:
> Kees,
>
>
> On 08/27/2014 02:46 AM, Will Deacon wrote:
>>
>> On Fri, Aug 22, 2014 at 01:19:13AM +0100, AKASHI Takahiro wrote:
>>>
>>> On 08/22/2014 01:47 AM, Kees Cook wrote:
>>>>
>>>> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
>>>> <takahiro.akashi@linaro.org> wrote:
>>>>>
>>>>> To allow tracer to be able to change/skip a system call by re-writing
>>>>> a syscall number, there are several approaches:
>>>>>
>>>>> (1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this
>>>>> case
>>>>> later on in syscall_trace_enter(), or
>>>>> (2) support ptrace(PTRACE_SET_SYSCALL) as on arm
>>>>>
>>>>> Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
>>>>> tracer as well as that secure_computing() expects a changed syscall
>>>>> number
>>>>> to be visible, especially case of -1, before this function returns in
>>>>> syscall_trace_enter(), we'd better take (2).
>>>>>
>>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
>>>>
>>>>
>>>> Thanks, I like having this on both arm and arm64.
>>>
>>>
>>> Yeah, having this simplified the code of syscall_trace_enter() a bit, but
>>> also imposes some restriction on arm64, too.
>>>
>>> > I wonder if other archs should add this option too.
>>>
>>> Do you think so? I assumed that SET_SYSCALL is to be avoided if possible.
>>>
>>> I also think that SET_SYSCALL should take an extra argument for a return
>>> value
>>> just in case of -1 (or we have SKIP_SYSCALL?).
>>
>>
>> I think we should propose this as a new request in the generic ptrace
>> code.
>> We can have an architecture-hook for actually setting the syscall, and
>> allow
>> architectures to define their own implementation of the request so they
>> can
>> be moved over one by one.
>
>
> What do you think about this request?

That sounds fine -- it doesn't need to be part of this series. I was
just noticing this was a common issue across multiple architectures.

-Kees

--
Kees Cook
Chrome OS Security