9p/trans_fd: Fix concurrency del of req_list in p9_fd_cancelled/p9_read_work

p9_read_work and p9_fd_cancelled may be called concurrently.
In some cases, req->req_list may be deleted by both p9_read_work
and p9_fd_cancelled.

We can fix it by ignoring replies associated with a cancelled
request and ignoring cancelled request if message has been received
before lock.

Link: http://lkml.kernel.org/r/20200612090833.36149-1-wanghai38@huawei.com
Fixes: 60ff779c ("9p: client: remove unused code and any reference to "cancelled" function")
Cc: <stable@vger.kernel.org> # v3.12+
Reported-by: syzbot+77a25acfa0382e06ab23@syzkaller.appspotmail.com
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_fd.c

@@ -362,6 +362,10 @@ static void p9_read_work(struct work_struct *work)
 		if (m->rreq->status == REQ_STATUS_SENT) {
 			list_del(&m->rreq->req_list);
 			p9_client_cb(m->client, m->rreq, REQ_STATUS_RCVD);
+		} else if (m->rreq->status == REQ_STATUS_FLSHD) {
+			/* Ignore replies associated with a cancelled request. */
+			p9_debug(P9_DEBUG_TRANS,
+				 "Ignore replies associated with a cancelled request\n");
 		} else {
 			spin_unlock(&m->client->lock);
 			p9_debug(P9_DEBUG_ERROR,
@@ -703,11 +707,20 @@ static int p9_fd_cancelled(struct p9_client *client, struct p9_req_t *req)
 {
 	p9_debug(P9_DEBUG_TRANS, "client %p req %p\n", client, req);
 
+	spin_lock(&client->lock);
+	/* Ignore cancelled request if message has been received
+	 * before lock.
+	 */
+	if (req->status == REQ_STATUS_RCVD) {
+		spin_unlock(&client->lock);
+		return 0;
+	}
+
 	/* we haven't received a response for oldreq,
 	 * remove it from the list.
 	 */
-	spin_lock(&client->lock);
 	list_del(&req->req_list);
+	req->status = REQ_STATUS_FLSHD;
 	spin_unlock(&client->lock);
 	p9_req_put(req);