Re: System-wide hard RLIMIT_STACK in 4.14.4+ w/ SELinux

[Laura Abbott]

On 12/12/2017 11:23 AM, Kees Cook wrote:
> On Tue, Dec 12, 2017 at 2:58 AM, TomÃÅ Trnka <trnka@xxxxxxx> wrote:
> > Hello,
> >
> > Commit 04e35f4495dd560db30c25efca4eecae8ec8c375 "exec: avoid RLIMIT_STACK
> > races with prlimit()" that made it into 4.14.4 effectively changes the default
> > hard RLIMIT_STACK on machines with SELinux (seen on Fedora 27).
> >
> > selinux_bprm_set_creds() sets bprm->secureexec for any SELinux domain
> > transition that does not have the "noatsecure" permission. The secureexec
> > logic thus kicks in for virtually every process launched by PID 1 systemd
> > (init_t), including gettys, display managers, etc.
>
> Uuugh. Okay, we need to revert that commit. I'll send a patch for 4.15
> (with a fix for -stable too).
>
> I will design an alternative, which was considered much earlier:
> keeping a copy of the rlimits in the bprm during exec so it can't
> change out from under the execing process. This will avoid needing to
> set the hard limit, avoid the locking race that commit was trying to
> fix, etc.
>
> This is an interesting state for the system to be in, though, it means
> AT_SECURE is being set for virtually all processes too? I would expect
> that might break a lot too (but clearly it hasn't).
>
> > I can see that 8 MiB "should be enough for everyone" using normal software,
> > but sadly the HPC stuff around here tends to need a little more (due to a
> > deficiency in gfortran).
> >
> > Minimal example (the actual types are not too important):
> >
> > # /bin/ulimit -Hs
> > unlimited
> > # runcon -r system_r -t sysadm_t runcon -t rpm_script_t /bin/ulimit -Hs
> > 8192
> >
> > Of course this can be somewhat worked around by adjusting the SELinux policy
> > (allowing blanket noatsecure permission for init_t and possibly others) or by
> > pam_limits (for components using PAM). Unfortunately, systemd's LimitSTACK= is
> > also broken (calls setrlimit before exec). Anyway, I wasn't expecting any of
> > that in connection with the 4.14.3->.4 upgrade.
> >
> > --
> > Best regards,
> >
> > TomÃÅ Trnka
> > Software for Chemistry & Materials
>
> Thanks for the report and examples!
>
> -Kees

FWIW, the issue I reported offline yesterday
https://bugzilla.redhat.com/show_bug.cgi?id=1524083 still happens with
selinux disabled. The conclusion there is still that trafficserver
needs to be fixed.

Thanks,
Laura