[PATCH] net/p9/trans_fd.c: fix double list_del()

[Tomas Bortoli]

A double list_del(&req->req_list) is possible in p9_fd_cancel() as
shown by Syzbot. To prevent it we have to ensure that we have the
client->lock when deleting the list. Furthermore, we have to update
the status of the request before releasing the lock, to prevent the
race.

Signed-off-by: Tomas Bortoli <tomasbortoli@xxxxxxxxx>
Reported-by: syzbot+735d926e9d1317c3310c@xxxxxxxxxxxxxxxxxxxxxxxxx
---
 net/9p/trans_fd.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index a64b01c56e30..370c6c69a05c 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -199,15 +199,14 @@ static void p9_mux_poll_stop(struct p9_conn *m)
 static void p9_conn_cancel(struct p9_conn *m, int err)
 {
 	struct p9_req_t *req, *rtmp;
-	unsigned long flags;
 	LIST_HEAD(cancel_list);
 
 	p9_debug(P9_DEBUG_ERROR, "mux %p err %d\n", m, err);
 
-	spin_lock_irqsave(&m->client->lock, flags);
+	spin_lock(&m->client->lock);
 
 	if (m->err) {
-		spin_unlock_irqrestore(&m->client->lock, flags);
+		spin_unlock(&m->client->lock);
 		return;
 	}
 
@@ -219,7 +218,6 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 	list_for_each_entry_safe(req, rtmp, &m->unsent_req_list, req_list) {
 		list_move(&req->req_list, &cancel_list);
 	}
-	spin_unlock_irqrestore(&m->client->lock, flags);
 
 	list_for_each_entry_safe(req, rtmp, &cancel_list, req_list) {
 		p9_debug(P9_DEBUG_ERROR, "call back req %p\n", req);
@@ -228,6 +226,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 			req->t_err = err;
 		p9_client_cb(m->client, req, REQ_STATUS_ERROR);
 	}
+	spin_unlock(&m->client->lock);
 }
 
 static __poll_t
@@ -370,12 +369,12 @@ static void p9_read_work(struct work_struct *work)
 		if (m->req->status != REQ_STATUS_ERROR)
 			status = REQ_STATUS_RCVD;
 		list_del(&m->req->req_list);
-		spin_unlock(&m->client->lock);
 		p9_client_cb(m->client, m->req, status);
 		m->rc.sdata = NULL;
 		m->rc.offset = 0;
 		m->rc.capacity = 0;
 		m->req = NULL;
+		spin_unlock(&m->client->lock);
 	}
 
 end_clear:
-- 
2.11.0