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

Josh Poimboeuf
2015-12-01T01:11:44Z

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.

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

--
Josh