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

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Kees Cook (keescook@chromium.org):
> On Wed, Nov 29, 2017 at 10:20 AM, Serge E. Hallyn <serge@hallyn.com> wrote:
> > Quoting Kees Cook (keescook@chromium.org):
> >> 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@codethink.co.uk>
> >> Reported-by: Brad Spengler <spender@grsecurity.net>
> >> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
> >> Cc: stable@vger.kernel.org
> >> Cc: James Morris <james.l.morris@oracle.com>
> >> Cc: Serge Hallyn <serge@hallyn.com>
> >
> > Acked-by: Serge Hallyn <serge@hallyn.com>
> 
> 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.

Oh, right, and so if soft > hard that will just end up ignored...  ok.