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

[Kees Cook]

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

-- 
Kees Cook
Pixel Security