[PATCH] net/9p/trans_fd.c: fix race by holding the lock

[Tomas Bortoli <tomasbortoli@gmail.com>]

It may be possible to run p9_fd_cancel() with a deleted req->req_list
and incur in a double del. To fix hold the client->lock while changing
the status, so the other threads will be synchronized.

Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Reported-by: syzbot+735d926e9d1317c3310c@syzkaller.appspotmail.com
---
 net/9p/trans_fd.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index 588bf88c3305..f9f96d50d96d 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -197,15 +197,14 @@ 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;
 	}

@@ -217,7 +216,6 @@ 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);
@@ -226,6 +224,7 @@ 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
@@ -373,8 +372,9 @@ 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);
+		/* update req->status while holding client->lock  */
 		p9_client_cb(m->client, m->req, status);
+		spin_unlock(&m->client->lock);
 		m->rc.sdata = NULL;
 		m->rc.offset = 0;
 		m->rc.capacity = 0;
--
2.11.0