Re: [PATCH v7 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

[Kees Cook]

On Wed, Oct 8, 2014 at 9:13 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> Hi Akashi,
>
> On Thu, Oct 02, 2014 at 10:46:11AM +0100, AKASHI Takahiro 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).
>>
>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/uapi/asm/ptrace.h |    1 +
>>  arch/arm64/kernel/ptrace.c           |   14 +++++++++++++-
>>  2 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
>> index 6913643..49c6174 100644
>> --- a/arch/arm64/include/uapi/asm/ptrace.h
>> +++ b/arch/arm64/include/uapi/asm/ptrace.h
>> @@ -23,6 +23,7 @@
>>
>>  #include <asm/hwcap.h>
>>
>> +#define PTRACE_SET_SYSCALL   23
>>
>>  /*
>>   * PSR bits
>> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
>> index fe63ac5..2842f9f 100644
>> --- a/arch/arm64/kernel/ptrace.c
>> +++ b/arch/arm64/kernel/ptrace.c
>> @@ -1082,7 +1082,19 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
>>  long arch_ptrace(struct task_struct *child, long request,
>>                unsigned long addr, unsigned long data)
>>  {
>> -     return ptrace_request(child, request, addr, data);
>> +     int ret;
>> +
>> +     switch (request) {
>> +             case PTRACE_SET_SYSCALL:
>> +                     task_pt_regs(child)->syscallno = data;
>> +                     ret = 0;
>> +                     break;
>> +             default:
>> +                     ret = ptrace_request(child, request, addr, data);
>> +                     break;
>> +     }
>> +
>> +     return ret;
>>  }
>
> I still don't understand why this needs to be in arch-specific code. Can't
> we implement this in generic code and get architectures to implement
> something like syscall_set_nr if they want the generic interface?

Personally, I'd rather see this land as-is in the arm64 tree, and then
later optimize PTRACE_SET_SYSCALL out of arm/ and arm64/, especially
since only these architectures implement this at the moment.

This is my plan for the asm-generic seccomp.h too -- I'd rather avoid
touching other architectures in this series, as it's easier to review
this way. Then we can optimize the code in a separate series, which
will have those changes isolated, etc.

-Kees

-- 
Kees Cook
Chrome OS Security
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