[PATCH] selinux: reorder hooks to make runtime disable less broken

[Casey Schaufler]

On 12/10/2019 3:27 AM, Ondrej Mosnacek wrote:
> On Mon, Dec 9, 2019 at 6:20 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
>> 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.
> On my way to lunch I came up with another relatively simple solution
> that should address this problem at the infrastructure level. Let me
> try to write it up into a patch, hopefully it will work...

I await your proposal with keen interest.

>
>> ---
>> * I do not plan to propose an implementation of loadable modules.
>>   I leave that as an exercise for the next generation.
>>
>>