Re: [PATCH] livepatch: fix race between enabled_store() and klp_unregister_patch()

libin
2015-12-01T02:46:53Z

Hi Josh,
on 2015/12/1 9:11, Josh Poimboeuf wrote:
> On Mon, Nov 30, 2015 at 11:54:37AM +0800, Li Bin wrote:
>> There is a potential race as following:
>>
>> CPU0 | CPU1
>> -----------------------------|-----------------------------------
>> enabled_store() | klp_unregister_patch()
>> | |-mutex_lock(&klp_mutex);
>> |-mutex_lock(&klp_mutex); | |-klp_free_patch();
>> | |-mutex_unlock(&klp_mutex);
>> |-[process the patch's state]|
>> |-mutex_unlock(&klp_mutex) |
>>
>> Fix this race condition by adding klp_is_patch_registered() check in
>> enabled_store() after get the lock klp_mutex.
> I'm thinking this race isn't possible, and that instead it would
> deadlock.

Good point. I did not consider the kernfs lock.

> When I try to recreate something similar by putting a delay in
> enabled_store(), klp_free_patch() just sleeps on its call to
> kobject_put() until enabled_store() returns. The unregister stack looks
> like:
>
> [<ffffffff812e966b>] __kernfs_remove+0x1fb/0x380
> [<ffffffff812ea273>] kernfs_remove+0x23/0x40
> [<ffffffff812ec601>] sysfs_remove_dir+0x51/0x80
> [<ffffffff81407fb8>] kobject_del+0x18/0x50
> [<ffffffff8140804a>] kobject_release+0x5a/0x190
> [<ffffffff81407f27>] kobject_put+0x27/0x50
> [<ffffffff81128d41>] klp_unregister_patch+0x71/0x80
>
> It seems to be waiting on a lock which is held by the kernfs code which
> called enabled_store(). So when enabled_store() tries to get the
> klp_mutex, it deadlocks.
>
> Miroslav and I previously discussed a few options to fix this:
>
> https://lkml.kernel.org/r/alpine.LNX.2.00.1502171728520.29490@pobox.suse.cz

Both the options seem good to me. I look forward to the patch. :)
Thanks,
Li Bin