[PATCH net] nfc: avoid potential race condition

[Dan Carpenter <dan.carpenter@oracle.com>]

This from static analysis inspired by CVE-2021-26708 where there was a
race condition because it didn't lock_sock(sk) before saving
"vsk->transport".  Here it is saving "llcp_sock->local" but the concept
is the same that it needs to take the lock first.

Fixes: 00e856db49bb ("NFC: llcp: Fall back to local values when getting socket options")
Fixes: d646960f7986 ("NFC: Initial LLCP support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
 net/nfc/llcp_sock.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index 6cfd30fc0798..74f4209c7144 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -314,14 +314,16 @@ static int nfc_llcp_getsockopt(struct socket *sock, int level, int optname,
 	if (get_user(len, optlen))
 		return -EFAULT;
 
-	local = llcp_sock->local;
-	if (!local)
-		return -ENODEV;
-
 	len = min_t(u32, len, sizeof(u32));
 
 	lock_sock(sk);
 
+	local = llcp_sock->local;
+	if (!local) {
+		release_sock(sk);
+		return -ENODEV;
+	}
+
 	switch (optname) {
 	case NFC_LLCP_RW:
 		rw = llcp_sock->rw > LLCP_MAX_RW ? local->rw : llcp_sock->rw;
@@ -598,14 +600,15 @@ static int llcp_sock_release(struct socket *sock)
 
 	pr_debug("%p\n", sk);
 
+	lock_sock(sk);
+
 	local = llcp_sock->local;
 	if (local == NULL) {
+		release_sock(sk);
 		err = -ENODEV;
 		goto out;
 	}
 
-	lock_sock(sk);
-
 	/* Send a DISC */
 	if (sk->sk_state == LLCP_CONNECTED)
 		nfc_llcp_send_disconnect(llcp_sock);
-- 
2.20.1