net: Fix TOCTOU issue in sk_is_readable()

[ Upstream commit 2660a544fdc0940bba15f70508a46cf9a6491230 ]

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: 8934ce2fd081 ("bpf: sockmap redirect ingress support")
Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250609-skisreadable-toctou-v1-1-d0dfb2d62c37@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/net/sock.h b/include/net/sock.h
index e716b2ba00bba8850028b3391de2f3fc0daa27db..e00cd9d0bec8e54bdd7cc0b821911af10016e8cc 100644 (file)
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -3061,8 +3061,11 @@ int sock_copy_user_timeval(struct __kernel_sock_timeval *tv,
 
 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 */