Subject: [PATCH net] ax25: use after free in ax25_connect

From: Hangyu Hua <[email-masked]>
Date: Tue, 11 Jan 2022 12:20:48 +0800

sk_to_ax25(sk) needs to be called after lock_sock(sk) to avoid UAF
caused by a race condition.

Signed-off-by: Hangyu Hua <[email-masked]>
---
 net/ax25/af_ax25.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index cfca99e295b8..c5d62420a2a8 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -1127,7 +1127,7 @@ static int __must_check ax25_connect(struct socket *sock,
 	struct sockaddr *uaddr, int addr_len, int flags)
 {
 	struct sock *sk = sock->sk;
-	ax25_cb *ax25 = sk_to_ax25(sk), *ax25t;
+	ax25_cb *ax25, *ax25t;
 	struct full_sockaddr_ax25 *fsa = (struct full_sockaddr_ax25 *)uaddr;
 	ax25_digi *digi = NULL;
 	int ct = 0, err = 0;
@@ -1155,6 +1155,8 @@ static int __must_check ax25_connect(struct socket *sock,
 
 	lock_sock(sk);
 
+	ax25 = sk_to_ax25(sk);
+
 	/* deal with restarts */
 	if (sock->state == SS_CONNECTING) {
 		switch (sk->sk_state) {
-- 
2.25.1