tls: skip setting sk_write_space on rekey

syzbot reported a problem when calling setsockopt(SO_SNDBUF) after a
rekey. SO_SNDBUF calls sk_write_space, ie tls_write_space, which then
calls the original socket's sk_write_space, saved in
ctx->sk_write_space. Rekeys should skip re-assigning
ctx->sk_write_space, so we don't end up with tls_write_space calling
itself.

Fixes: 4706959 ("tls: implement rekey for TLS1.3")
Reported-by: [email protected]
Closes: https://lore.kernel.org/netdev/[email protected]/
Tested-by: [email protected]
Signed-off-by: Sabrina Dubroca <[email protected]>
Link: https://patch.msgid.link/ffdbe4de691d1c1eead556bbf42e33ae215304a7.1736436785.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <[email protected]>

Author: qsn

net/tls/tls_main.c

@@ -737,6 +737,10 @@ static int do_tls_setsockopt_conf(struct sock *sk, sockptr_t optval,
 	else
 		ctx->rx_conf = conf;
 	update_sk_prot(sk, ctx);
+
+	if (update)
+		return 0;
+
 	if (tx) {
 		ctx->sk_write_space = sk->sk_write_space;
 		sk->sk_write_space = tls_write_space;