Re: [PATCH v2 1/2] IMA: Define workqueue for early boot "key" measurements

Mimi Zohar
2019-12-12T22:54:28Z

On Thu, 2019-12-12 at 13:59 -0800, Lakshmi Ramasubramanian wrote:
> On 12/12/19 1:13 PM, Mimi Zohar wrote:
>
> >
> > Looking at this again, something seems off or at least the comment
> > doesn't match the code.
> >
> > /*
> > * To avoid holding the mutex while processing queued keys,
> > * transfer the queued keys with the mutex held to a temp list,
> > * release the mutex, and then process the queued keys from
> > * the temp list.
> > *
> > * Since ima_process_keys is set to true above, any new key will
> > * be processed immediately and not queued.
> > */
> >
> > Setting ima_process_key before taking the lock won't prevent the race.
> >  I think you want to test ima_process_keys before taking the lock and
> > again immediately afterward taking the lock, before setting it.  Then
> > the comment would match the code.
> >
> > Shouldn't ima_process_keys be defined as static to limit the scope to
> > this file?
> >
> > Mimi
> >
>
> In IMA hook, ima_process_key is checked without lock. If it is false,
> ima_queue_key is called. If the key was queued (by ima_queue_key()) then
> the hook defers measurement. Else, it processes it immediately.
>
> In ima_queue_key() function the check for ima_process_key is done after
> taking the lock and the key queued if the flag is false.
>
> In ima_process_keys() ima_process_key is set without lock and then the
> queued keys are moved to a temp list after taking the lock.
>
> I have reviewed the changes myself and also with a few of my colleagues.
> I don't think there is a race condition. Please let me know if you do
> see a problem.
>
> I can move the setting of ima_process_key flag inside the lock. But
> honestly I don't think that is necessary.
>
> I agree that ima_process_keys should be static since it is used in this
> file one. I'll make that change.
>
> I can also move the setting of ima_process_key flag inside the lock
> along with the above change.

My concern is with the last sentence "Since ima_process_keys is set to
true above, any new key will be processed immediately and not queued."
It's unlikely, but possible, that a second process will wait for the
ima_keys_mutex.  Either we remove this sentence or move setting
ima_process_keys to after taking the lock.

Mimi