linux-kernel - Re: System-wide hard RLIMIT

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Date:   Tue, 12 Dec 2017 11:52:57 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Kees Cook <keescook@...omium.org>,
        Tomáš Trnka <trnka@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <pmoore@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: System-wide hard RLIMIT_STACK in 4.14.4+ w/ SELinux

On 12/12/2017 11:23 AM, Kees Cook wrote:
> On Tue, Dec 12, 2017 at 2:58 AM, Tomáš Trnka <trnka@....com> 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,
>>
>> Tomáš 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