On Wed, Nov 29, 2017 at 10:20 AM, Serge E. Hallyn <[email protected]> wrote:
> Quoting Kees Cook ([email protected]):
>> While the defense-in-depth RLIMIT_STACK limit on setuid processes was
>> protected against races from other threads calling setrlimit(), I missed
>> protecting it against races from external processes calling prlimit().
>> This adds locking around the change and makes sure that rlim_max is set
>> too.
>>
>> Reported-by: Ben Hutchings <[email protected]>
>> Reported-by: Brad Spengler <[email protected]>
>> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
>> Cc: [email protected]
>> Cc: James Morris <[email protected]>
>> Cc: Serge Hallyn <[email protected]>
>
> Acked-by: Serge Hallyn <[email protected]>
Thanks!
>
> The only thing i'm wondering is in do_prlimit():
>
> . 1480 if (new_rlim) {
> . 1481 if (new_rlim->rlim_cur > new_rlim->rlim_max)
> . 1482 return -EINVAL;
>
> that bit is done not under the lock. Does that still allow a
> race, if this check is done before the below block, and then the
> rest proceeds after?
>
> I *think* not, because later in do_prlimit() it will return -EPERM if
>
> . 1500 if (new_rlim->rlim_max > rlim->rlim_max &&
> . 1501 !capable(CAP_SYS_RESOURCE))
>
> Although rlim is gathered before the lock, but that is a struct *
> so should be ok?
I stared at this for a while too. I think it's okay because the max is
checked under the lock, so the max can't be raced to be raised. The
cur value could get raced, though, but I don't think that's a problem,
since it's the "soft" limit.
-Kees
--
Kees Cook
Pixel Security
