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

From: Laura Abbott
Date: Tue Dec 12 2017 - 15:10:43 EST


On 12/12/2017 11:56 AM, Kees Cook wrote:
On Tue, Dec 12, 2017 at 11:52 AM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
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.

I've sent a revert regardless. I think the bprm needs to hold a copy
of the rlimits so they can't be manipulated during exec. This will
keep the hardlimit from being messed with, etc.

-Kees


Understood. I just wanted to clarify that the issues were separate
since there was still some question about where the secureexec
was coming from.

Thanks,
Laura