tls: skip setting sk_write_space on rekey

JIRA: https://issues.redhat.com/browse/RHEL-86020

commit 06cc878
Author: Sabrina Dubroca <sd@queasysnail.net>
Date:   Thu Jan 9 23:30:54 2025 +0100

    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: syzbot+6ac73b3abf1b598863fa@syzkaller.appspotmail.com
    Closes: https://lore.kernel.org/netdev/676d231b.050a0220.2f3838.0461.GAE@google.com/
    Tested-by: syzbot+6ac73b3abf1b598863fa@syzkaller.appspotmail.com
    Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
    Link: https://patch.msgid.link/ffdbe4de691d1c1eead556bbf42e33ae215304a7.1736436785.git.sd@queasysnail.net
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Sabrina Dubroca <sdubroca@redhat.com>

Author: Sabrina Dubroca

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;