[PATCH net v1] net/rose: fix NULL pointer dereference in rose_transmit_link on reconnect

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

syzkaller reported a bug [1], and the reproducer is available at [2].

When rose_connect() is called a second time on an already-connecting
socket, it overwrites rose->neighbour with the result of rose_get_neigh()
without releasing the previous reference. If rose_get_neigh() returns
NULL, the socket is left in an inconsistent state: rose->state remains
ROSE_STATE_1 from the first connect while rose->neighbour is NULL.

When the socket is subsequently closed, rose_release() sees ROSE_STATE_1
and calls rose_write_internal() -> rose_transmit_link(skb, NULL), causing
a NULL pointer dereference when accessing neigh->loopback.

Fix this by:
1. Releasing the old neighbour reference before attempting a reconnect
2. Resetting rose->state to ROSE_STATE_0 before the new connect attempt,
   so a failure leaves the socket in a clean state
3. Setting rose->neighbour to NULL in all error paths after
   rose_neigh_put() to prevent use-after-free on subsequent reconnects

[1] https://syzkaller.appspot.com/bug?extid=d00f90e0af54102fb271
[2] https://gist.github.com/mrpre/9e6779e0d13e2c66779b1653fef80516

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+d00f90e0af54102fb271@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69694d6f.050a0220.58bed.0027.GAE@google.com/T/
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 net/rose/af_rose.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/rose/af_rose.c b/net/rose/af_rose.c
index 841d62481048..d9bf32ac3df3 100644
--- a/net/rose/af_rose.c
+++ b/net/rose/af_rose.c
@@ -814,6 +814,14 @@ static int rose_connect(struct socket *sock, struct sockaddr_unsized *uaddr, int
 	sk->sk_state   = TCP_CLOSE;
 	sock->state = SS_UNCONNECTED;
 
+	/* Release previous neighbour ref if reconnecting */
+	if (rose->neighbour) {
+		rose_neigh_put(rose->neighbour);
+		rose->neighbour = NULL;
+	}
+
+	rose->state = ROSE_STATE_0;
+
 	rose->neighbour = rose_get_neigh(&addr->srose_addr, &cause,
 					 &diagnostic, 0);
 	if (!rose->neighbour) {
@@ -825,6 +833,7 @@ static int rose_connect(struct socket *sock, struct sockaddr_unsized *uaddr, int
 	if (!rose->lci) {
 		err = -ENETUNREACH;
 		rose_neigh_put(rose->neighbour);
+		rose->neighbour = NULL;
 		goto out_release;
 	}
 
@@ -837,6 +846,7 @@ static int rose_connect(struct socket *sock, struct sockaddr_unsized *uaddr, int
 		if (!dev) {
 			err = -ENETUNREACH;
 			rose_neigh_put(rose->neighbour);
+			rose->neighbour = NULL;
 			goto out_release;
 		}
 
@@ -844,6 +854,7 @@ static int rose_connect(struct socket *sock, struct sockaddr_unsized *uaddr, int
 		if (!user) {
 			err = -EINVAL;
 			rose_neigh_put(rose->neighbour);
+			rose->neighbour = NULL;
 			dev_put(dev);
 			goto out_release;
 		}
-- 
2.43.0