net/9p: use a dedicated spinlock for trans_fd

Shamelessly copying the explanation from Tetsuo Handa's suggested
patch[1] (slightly reworded):
syzbot is reporting inconsistent lock state in p9_req_put()[2],
for p9_tag_remove() from p9_req_put() from IRQ context is using
spin_lock_irqsave() on "struct p9_client"->lock but trans_fd
(not from IRQ context) is using spin_lock().

Since the locks actually protect different things in client.c and in
trans_fd.c, just replace trans_fd.c's lock by a new one specific to the
transport (client.c's protect the idr for fid/tag allocations,
while trans_fd.c's protects its own req list and request status field
that acts as the transport's state machine)

Link: https://lore.kernel.org/r/20220904112928.1308799-1-asmadeus@codewreck.org
Link: https://lkml.kernel.org/r/2470e028-9b05-2013-7198-1fdad071d999@I-love.SAKURA.ne.jp [1]
Link: https://syzkaller.appspot.com/bug?extid=2f20b523930c32c160cc [2]
Reported-by: syzbot <syzbot+2f20b523930c32c160cc@syzkaller.appspotmail.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_fd.c

@@ -91,6 +91,7 @@ struct p9_poll_wait {
  * @mux_list: list link for mux to manage multiple connections (?)
  * @client: reference to client instance for this connection
  * @err: error state
+ * @req_lock: lock protecting req_list and requests statuses
  * @req_list: accounting for requests which have been sent
  * @unsent_req_list: accounting for requests that haven't been sent
  * @rreq: read request
@@ -114,6 +115,7 @@ struct p9_conn {
 	struct list_head mux_list;
 	struct p9_client *client;
 	int err;
+	spinlock_t req_lock;
 	struct list_head req_list;
 	struct list_head unsent_req_list;
 	struct p9_req_t *rreq;
@@ -189,10 +191,10 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 
 	p9_debug(P9_DEBUG_ERROR, "mux %p err %d\n", m, err);
 
-	spin_lock(&m->client->lock);
+	spin_lock(&m->req_lock);
 
 	if (m->err) {
-		spin_unlock(&m->client->lock);
+		spin_unlock(&m->req_lock);
 		return;
 	}
 
@@ -205,7 +207,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 		list_move(&req->req_list, &cancel_list);
 	}
 
-	spin_unlock(&m->client->lock);
+	spin_unlock(&m->req_lock);
 
 	list_for_each_entry_safe(req, rtmp, &cancel_list, req_list) {
 		p9_debug(P9_DEBUG_ERROR, "call back req %p\n", req);
@@ -360,7 +362,7 @@ static void p9_read_work(struct work_struct *work)
 	if ((m->rreq) && (m->rc.offset == m->rc.capacity)) {
 		p9_debug(P9_DEBUG_TRANS, "got new packet\n");
 		m->rreq->rc.size = m->rc.offset;
-		spin_lock(&m->client->lock);
+		spin_lock(&m->req_lock);
 		if (m->rreq->status == REQ_STATUS_SENT) {
 			list_del(&m->rreq->req_list);
 			p9_client_cb(m->client, m->rreq, REQ_STATUS_RCVD);
@@ -369,14 +371,14 @@ static void p9_read_work(struct work_struct *work)
 			p9_debug(P9_DEBUG_TRANS,
 				 "Ignore replies associated with a cancelled request\n");
 		} else {
-			spin_unlock(&m->client->lock);
+			spin_unlock(&m->req_lock);
 			p9_debug(P9_DEBUG_ERROR,
 				 "Request tag %d errored out while we were reading the reply\n",
 				 m->rc.tag);
 			err = -EIO;
 			goto error;
 		}
-		spin_unlock(&m->client->lock);
+		spin_unlock(&m->req_lock);
 		m->rc.sdata = NULL;
 		m->rc.offset = 0;
 		m->rc.capacity = 0;
@@ -454,10 +456,10 @@ static void p9_write_work(struct work_struct *work)
 	}
 
 	if (!m->wsize) {
-		spin_lock(&m->client->lock);
+		spin_lock(&m->req_lock);
 		if (list_empty(&m->unsent_req_list)) {
 			clear_bit(Wworksched, &m->wsched);
-			spin_unlock(&m->client->lock);
+			spin_unlock(&m->req_lock);
 			return;
 		}
 
@@ -472,7 +474,7 @@ static void p9_write_work(struct work_struct *work)
 		m->wpos = 0;
 		p9_req_get(req);
 		m->wreq = req;
-		spin_unlock(&m->client->lock);
+		spin_unlock(&m->req_lock);
 	}
 
 	p9_debug(P9_DEBUG_TRANS, "mux %p pos %d size %d\n",
@@ -589,6 +591,7 @@ static void p9_conn_create(struct p9_client *client)
 	INIT_LIST_HEAD(&m->mux_list);
 	m->client = client;
 
+	spin_lock_init(&m->req_lock);
 	INIT_LIST_HEAD(&m->req_list);
 	INIT_LIST_HEAD(&m->unsent_req_list);
 	INIT_WORK(&m->rq, p9_read_work);
@@ -670,10 +673,10 @@ static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 	if (m->err < 0)
 		return m->err;
 
-	spin_lock(&client->lock);
+	spin_lock(&m->req_lock);
 	req->status = REQ_STATUS_UNSENT;
 	list_add_tail(&req->req_list, &m->unsent_req_list);
-	spin_unlock(&client->lock);
+	spin_unlock(&m->req_lock);
 
 	if (test_and_clear_bit(Wpending, &m->wsched))
 		n = EPOLLOUT;
@@ -688,11 +691,13 @@ static int p9_fd_request(struct p9_client *client, struct p9_req_t *req)
 
 static int p9_fd_cancel(struct p9_client *client, struct p9_req_t *req)
 {
+	struct p9_trans_fd *ts = client->trans;
+	struct p9_conn *m = &ts->conn;
 	int ret = 1;
 
 	p9_debug(P9_DEBUG_TRANS, "client %p req %p\n", client, req);
 
-	spin_lock(&client->lock);
+	spin_lock(&m->req_lock);
 
 	if (req->status == REQ_STATUS_UNSENT) {
 		list_del(&req->req_list);
@@ -700,21 +705,24 @@ static int p9_fd_cancel(struct p9_client *client, struct p9_req_t *req)
 		p9_req_put(client, req);
 		ret = 0;
 	}
-	spin_unlock(&client->lock);
+	spin_unlock(&m->req_lock);
 
 	return ret;
 }
 
 static int p9_fd_cancelled(struct p9_client *client, struct p9_req_t *req)
 {
+	struct p9_trans_fd *ts = client->trans;
+	struct p9_conn *m = &ts->conn;
+
 	p9_debug(P9_DEBUG_TRANS, "client %p req %p\n", client, req);
 
-	spin_lock(&client->lock);
+	spin_lock(&m->req_lock);
 	/* Ignore cancelled request if message has been received
 	 * before lock.
 	 */
 	if (req->status == REQ_STATUS_RCVD) {
-		spin_unlock(&client->lock);
+		spin_unlock(&m->req_lock);
 		return 0;
 	}
 
@@ -723,7 +731,8 @@ static int p9_fd_cancelled(struct p9_client *client, struct p9_req_t *req)
 	 */
 	list_del(&req->req_list);
 	req->status = REQ_STATUS_FLSHD;
-	spin_unlock(&client->lock);
+	spin_unlock(&m->req_lock);
+
 	p9_req_put(client, req);
 
 	return 0;