From: Casey Schaufler <casey@schaufler-ca.com>
To: Stephen Smalley <sds@tycho.nsa.gov>,
Ondrej Mosnacek <omosnace@redhat.com>,
selinux@vger.kernel.org, Paul Moore <paul@paul-moore.com>,
LSM <linux-security-module@vger.kernel.org>,
Casey Schaufler <casey@schaufler-ca.com>
Subject: Re: [PATCH] selinux: reorder hooks to make runtime disable less broken
Date: Mon, 9 Dec 2019 09:20:25 -0800 [thread overview]
Message-ID: <2fadcd3d-96d2-82bf-f221-a7961853be50@schaufler-ca.com> (raw)
In-Reply-To: <ecfd3846-b38f-4b85-4568-d64625c490ac@tycho.nsa.gov>
On 12/9/2019 5:58 AM, Stephen Smalley wrote:
> On 12/9/19 8:21 AM, Stephen Smalley wrote:
>> On 12/9/19 2:57 AM, Ondrej Mosnacek wrote:
>>> Commit b1d9e6b0646d ("LSM: Switch to lists of hooks") switched the LSM
>>> infrastructure to use per-hook lists, which meant that removing the
>>> hooks for a given module was no longer atomic. Even though the commit
>>> clearly documents that modules implementing runtime revmoval of hooks
>>> (only SELinux attempts this madness) need to take special precautions to
>>> avoid race conditions, SELinux has never addressed this.
>>>
>>> By inserting an artificial delay between the loop iterations of
>>> security_delete_hooks() (I used 100 ms), booting to a state where
>>> SELinux is enabled, but policy is not yet loaded, and running these
>>> commands:
>>>
>>> while true; do ping -c 1 <some IP>; done &
>>> echo -n 1 >/sys/fs/selinux/disable
>>> kill %1
>>> wait
>>>
>>> ...I was able to trigger NULL pointer dereferences in various places. I
>>> also have a report of someone getting panics on a stock RHEL-8 kernel
>>> after setting SELINUX=disabled in /etc/selinux/config and rebooting
>>> (without adding "selinux=0" to kernel command-line).
>>>
>>> Reordering the SELinux hooks such that those that allocate structures
>>> are removed last seems to prevent these panics. It is very much possible
>>> that this doesn't make the runtime disable completely race-free, but at
>>> least it makes the operation much less fragile.
>>>
>>> An alternative (and safer) solution would be to add NULL checks to each
>>> hook, but doing this just to support the runtime disable hack doesn't
>>> seem to be worth the effort...
>>
>> Personally, I would prefer to just get rid of runtime disable altogether; it also precludes making the hooks read-only after initialization. IMHO, selinux=0 is the proper way to disable SELinux if necessary. I believe there is an open bugzilla on Fedora related to this issue, since runtime disable was originally introduced for Fedora.
>
> Also, if we have to retain this support, it seems like this ought to be fixed in the LSM framework especially since it was a change there that broke the SELinux implementation.
Agreed, mostly. Deleting an LSM is fundamentally something the infrastructure
should handle *if* we allow it. Should we decide at some point to allow loadable
modules, as Tetsuo has advocated from time to time, we would need a general
solution. We don't have a general solution now because only SELinux wants it.
The previous implementation was under #ifdef for SELinux. At the time I understood
that there was no interest in investing in it. The implementation passed tests
at the time.
I propose that until such time as someone decides to seriously investigate
loadable security modules* the sole user of the deletion mechanism is
welcome to invest whatever they like in their special case, and I will be
happy to lend whatever assistance I can.
---
* I do not plan to propose an implementation of loadable modules.
I leave that as an exercise for the next generation.
next prev parent reply other threads:[~2019-12-09 17:20 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-09 7:57 [PATCH] selinux: reorder hooks to make runtime disable less broken Ondrej Mosnacek
2019-12-09 7:59 ` Ondrej Mosnacek
2019-12-09 13:21 ` Stephen Smalley
2019-12-09 13:58 ` Stephen Smalley
2019-12-09 17:20 ` Casey Schaufler [this message]
2019-12-10 11:27 ` Ondrej Mosnacek
2019-12-10 16:23 ` Casey Schaufler
2019-12-09 16:32 ` Casey Schaufler
[not found] ` <CAOSEQ1pHnxrMyn1qYXzJPaT6Smf1ycVOfHQ7-gkDpzYiq9S=Cg@mail.gmail.com>
2019-12-09 17:37 ` Casey Schaufler
2019-12-10 11:19 ` Ondrej Mosnacek
2019-12-10 19:29 ` Paul Moore
2019-12-10 19:43 ` Casey Schaufler
2019-12-10 19:50 ` Stephen Smalley
2019-12-10 19:57 ` Casey Schaufler
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