io-wq: check that the predecessor is hashed in io_wq_remove_pending()

io_wq_remove_pending() needs to fix up wq->hash_tail[] if the cancelled
work was the tail of its hash bucket. When doing this, it checks whether
the preceding entry in acct->work_list has the same hash value, but
never checks that the predecessor is hashed at all. io_get_work_hash()
is simply atomic_read(&work->flags) >> IO_WQ_HASH_SHIFT, and the hash
bits are never set for non-hashed work, so it returns 0. Thus, when a
hashed bucket-0 work is cancelled while a non-hashed work is its list
predecessor, the check spuriously passes and a pointer to the non-hashed
io_kiocb is stored in wq->hash_tail[0].

Because non-hashed work is dequeued via the fast path in
io_get_next_work(), which never touches hash_tail[], the stale pointer
is never cleared. Therefore, after the non-hashed io_kiocb completes and
is freed back to req_cachep, wq->hash_tail[0] is a dangling pointer. The
io_wq is per-task (tctx->io_wq) and survives ring open/close, so the
dangling pointer persists for the lifetime of the task; the next hashed
bucket-0 enqueue dereferences it in io_wq_insert_work() and
wq_list_add_after() writes through freed memory.

Add the missing io_wq_is_hashed() check so a non-hashed predecessor
never inherits a hash_tail[] slot.

Cc: stable@vger.kernel.org
Fixes: 204361a7 ("io-wq: fix hang after cancelling pending hashed work")
Signed-off-by: Nicholas Carlini <nicholas@carlini.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>