Messages in this thread | | | From | Kees Cook <> | Date | Wed, 29 Nov 2017 10:27:47 -0800 | Subject | Re: [PATCH] exec: Avoid RLIMIT_STACK races with prlimit() |
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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.
-Kees
-- Kees Cook Pixel Security
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