Re: [PATCH] exec: Avoid RLIMIT_STACK races with prlimit()

[Kees Cook]

On Wed, Nov 29, 2017 at 10:20 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Kees Cook (keescook@xxxxxxxxxxxx):
>> 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 <ben.hutchings@xxxxxxxxxxxxxxx>
>> Reported-by: Brad Spengler <spender@xxxxxxxxxxxxxx>
>> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Cc: James Morris <james.l.morris@xxxxxxxxxx>
>> Cc: Serge Hallyn <serge@xxxxxxxxxx>
>
> Acked-by: Serge Hallyn <serge@xxxxxxxxxx>

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