Re: [syzbot] KASAN: use-after-free Write in io_queue_worker_create

[Jens Axboe]

On 12/5/21 11:43 PM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    944207047ca4 Merge tag 'usb-5.16-rc4' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13ebd129b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=171728a464c05f2b
> dashboard link: https://syzkaller.appspot.com/bug?extid=b60c982cb0efc5e05a47
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b60c982cb0efc5e05a47@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> BUG: KASAN: use-after-free in instrument_atomic_write include/linux/instrumented.h:86 [inline]
> BUG: KASAN: use-after-free in clear_bit_unlock include/asm-generic/bitops/instrumented-lock.h:25 [inline]
> BUG: KASAN: use-after-free in io_queue_worker_create+0x453/0x4e0 fs/io-wq.c:363
> Write of size 8 at addr ffff888023e068d8 by task kworker/3:4/13798

Looks like a spurious clear that can race with the task_work already ran and
the worker got dropped. Both handlers do clear it, so I think we just need:

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 50cf9f92da36..35da9d90df76 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -359,10 +359,8 @@ static bool io_queue_worker_create(struct io_worker *worker,
 
 	init_task_work(&worker->create_work, func);
 	worker->create_index = acct->index;
-	if (!task_work_add(wq->task, &worker->create_work, TWA_SIGNAL)) {
-		clear_bit_unlock(0, &worker->create_state);
+	if (!task_work_add(wq->task, &worker->create_work, TWA_SIGNAL))
 		return true;
-	}
 	clear_bit_unlock(0, &worker->create_state);
 fail_release:
 	io_worker_release(worker);

-- 
Jens Axboe