Re: [patch] seccomp.2: Note that memory area is read-only

[Michael Kerrisk (man-pages)]

On 19 April 2015 at 01:37, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Mon, Apr 6, 2015 at 8:29 AM, Michael Kerrisk (man-pages)
> <mtk.manpages@xxxxxxxxx> wrote:
>> Hi Kees,
>>
>> I recently was asked about the point below, and had to go check the code
>> to be sure, since the man page said nothing. It would be good to have
>> a confirmation: the seccomp_data buffer supplied to a seccomp BPF program
>> is read-only, right? (That is, one can't write to the buffer in order to
>> change the arguments that a system call actually receives.)
>
> That's correct. If BPF even allows changing the data, it's not copied
> back to the syscall when it runs. Anything wanting to do things like
> that would need to use ptrace to catch the call an directly modify the
> registers before continuing with the call.
>
>>
>> A small man page patch below.
>
> Looks good, thanks!

Thanks, Kees!

Cheers,

Michael


>> --- a/man2/seccomp.2
>> +++ b/man2/seccomp.2
>> @@ -232,15 +232,15 @@ struct sock_filter {            /* Filter block */
>>  };
>>  .fi
>>  .in
>>
>>  When executing the instructions, the BPF program operates on the
>>  system call information made available (i.e., use the
>>  .BR BPF_ABS
>> -addressing mode) as a buffer of the following form:
>> +addressing mode) as a (read-only) buffer of the following form:
>>
>>  .in +4n
>>  .nf
>>  struct seccomp_data {
>>      int   nr;                   /* System call number */
>>      __u32 arch;                 /* AUDIT_ARCH_* value
>>                                     (see <linux/audit.h>) */
>>
>>
>> --
>> Michael Kerrisk
>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>> Linux/UNIX System Programming Training: http://man7.org/training/
>
>
>
> --
> Kees Cook
> Chrome OS Security



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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