net: Fix TOCTOU issue in sk_is_readable()

sk->sk_prot->sock_is_readable is a valid function pointer when sk resides
in a sockmap. After the last sk_psock_put() (which usually happens when
socket is removed from sockmap), sk->sk_prot gets restored and
sk->sk_prot->sock_is_readable becomes NULL.

This makes sk_is_readable() racy, if the value of sk->sk_prot is reloaded
after the initial check. Which in turn may lead to a null pointer
dereference.

Ensure the function pointer does not turn NULL after the check.

Fixes: 8934ce2 ("bpf: sockmap redirect ingress support")
Suggested-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Michal Luczaj <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: mmhal

include/net/sock.h

@@ -3010,8 +3010,11 @@ int sock_ioctl_inout(struct sock *sk, unsigned int cmd,
 int sk_ioctl(struct sock *sk, unsigned int cmd, void __user *arg);
 static inline bool sk_is_readable(struct sock *sk)
 {
-	if (sk->sk_prot->sock_is_readable)
-		return sk->sk_prot->sock_is_readable(sk);
+	const struct proto *prot = READ_ONCE(sk->sk_prot);
+
+	if (prot->sock_is_readable)
+		return prot->sock_is_readable(sk);
+
 	return false;
 }
 #endif	/* _SOCK_H */