io_uring: fix cancellation overwriting req->flags

Only the current owner of a request is allowed to write into req->flags.
Hence, the cancellation path should never touch it. Add a new field
instead of the flag, move it into the 3rd cache line because it should
always be initialised. poll_refs can move further as polling is an
involved process anyway.

It's a minimal patch, in the future we can and should find a better
place for it and remove now unused REQ_F_CANCEL_SEQ.

Fixes: 521223d7 ("io_uring/cancel: don't default to setting req->work.cancel_seq")
Cc: stable@vger.kernel.org
Reported-by: Li Shi <sl1589472800@gmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6827b129f8f0ad76fa9d1f0a773de938b240ffab.1718323430.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -648,7 +648,7 @@ struct io_kiocb {
 	struct io_rsrc_node		*rsrc_node;
 
 	atomic_t			refs;
-	atomic_t			poll_refs;
+	bool				cancel_seq_set;
 	struct io_task_work		io_task_work;
 	/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
 	struct hlist_node		hash_node;
@@ -657,6 +657,7 @@ struct io_kiocb {
 	/* opcode allocated if it needs to store data for async defer */
 	void				*async_data;
 	/* linked requests, IFF REQ_F_HARDLINK or REQ_F_LINK are set */
+	atomic_t			poll_refs;
 	struct io_kiocb			*link;
 	/* custom credentials, valid IFF REQ_F_CREDS is set */
 	const struct cred		*creds;

io_uring/cancel.h

@@ -27,10 +27,10 @@ bool io_cancel_req_match(struct io_kiocb *req, struct io_cancel_data *cd);
 
 static inline bool io_cancel_match_sequence(struct io_kiocb *req, int sequence)
 {
-	if ((req->flags & REQ_F_CANCEL_SEQ) && sequence == req->work.cancel_seq)
+	if (req->cancel_seq_set && sequence == req->work.cancel_seq)
 		return true;
 
-	req->flags |= REQ_F_CANCEL_SEQ;
+	req->cancel_seq_set = true;
 	req->work.cancel_seq = sequence;
 	return false;
 }

io_uring/io_uring.c

@@ -2058,6 +2058,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	req->file = NULL;
 	req->rsrc_node = NULL;
 	req->task = current;
+	req->cancel_seq_set = false;
 
 	if (unlikely(opcode >= IORING_OP_LAST)) {
 		req->opcode = 0;