[PATCH] Bluetooth: af_bluetooth: Fix Use-After-Free in bt_sock_recvmsg

[Hyunwoo Kim <v4bel@theori.io>]

This can cause a race with bt_sock_ioctl() because
bt_sock_recvmsg() gets the skb from sk->sk_receive_queue
and then frees it without holding lock_sock.
A use-after-free for a skb occurs with the following flow.
```
bt_sock_recvmsg() -> skb_recv_datagram() -> skb_free_datagram()
bt_sock_ioctl() -> skb_peek()
```
Add lock_sock to bt_sock_recvmsg() to fix this issue.

Signed-off-by: Hyunwoo Kim <v4bel@theori.io>
---
 net/bluetooth/af_bluetooth.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/af_bluetooth.c b/net/bluetooth/af_bluetooth.c
index 336a76165454..9def13ac816e 100644
--- a/net/bluetooth/af_bluetooth.c
+++ b/net/bluetooth/af_bluetooth.c
@@ -309,11 +309,16 @@ int bt_sock_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	if (flags & MSG_OOB)
 		return -EOPNOTSUPP;
 
+	lock_sock(sk);
+
 	skb = skb_recv_datagram(sk, flags, &err);
 	if (!skb) {
-		if (sk->sk_shutdown & RCV_SHUTDOWN)
+		if (sk->sk_shutdown & RCV_SHUTDOWN) {
+			release_sock(sk);
 			return 0;
+		}
 
+		release_sock(sk);
 		return err;
 	}
 
@@ -343,6 +348,8 @@ int bt_sock_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 
 	skb_free_datagram(sk, skb);
 
+	release_sock(sk);
+
 	if (flags & MSG_TRUNC)
 		copied = skblen;
 
-- 
2.25.1