io_uring/cmd: don't leave list poison in ->hash_node

io_uring_cmd_del_cancelable() uses hlist_del() to take a command off
->cancelable_uring_cmd, which leaves LIST_POISON1/2 behind in
->hash_node. Requests are recycled through the request cache without
being reinitialised, and not every user of ->hash_node sets it up before
it is used:

If a vectored futex wait is woken while futex_wait_multiple_setup() is
still queueing the futexes, io_futexv_wait() doesn't add the request to
->futex_list, and leaves the completion to the task_work queued by the
wake up. io_futexv_complete() ends in __io_futex_complete(), whose
hlist_del_init() is fine for a request that is unhashed, which is how all
other users of the node leave it. If the request was a cancelable
uring_cmd in its previous life, e.g. a ublk FETCH_REQ, it writes through
LIST_POISON2:

  Oops: general protection fault, probably for non-canonical address
  KASAN: maybe wild-memory-access in range [0xdead000000000120-...]
  RIP: 0010:io_futexv_complete+0x219/0x420
  Call Trace:
   tctx_task_work_run+0x804/0x950
   tctx_task_work+0x71/0xa0
   task_work_run+0x118/0x200

Leave the node unhashed.

Fixes: 93b8cc60c3 ("io_uring: cancelable uring_cmd")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://patch.msgid.link/20261015-b4-io-uring-fixes-v1-2-c1556f5d7bf5@toxicpanda.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/uring_cmd.c

@@ -87,7 +87,7 @@ static void io_uring_cmd_del_cancelable(struct io_uring_cmd *cmd,
 
 	cmd->flags &= ~IORING_URING_CMD_CANCELABLE;
 	io_ring_submit_lock(ctx, issue_flags);
-	hlist_del(&req->hash_node);
+	hlist_del_init(&req->hash_node);
 	io_ring_submit_unlock(ctx, issue_flags);
 }