net: tls: avoid hanging tasks on the tx_lock

syzbot sent a hung task report and Eric explains that adversarial
receiver may keep RWIN at 0 for a long time, so we are not guaranteed
to make forward progress. Thread which took tx_lock and went to sleep
may not release tx_lock for hours. Use interruptible sleep where
possible and reschedule the work if it can't take the lock.

Testing: existing selftest passes

Reported-by: [e-mail]
Fixes: 79ffe6087e91 ("net/tls: add a TX lock")
Link: https://lore.kernel.org/all/[e-mail]/
Cc: [e-mail] # wait 4 weeks
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/tls/tls_sw.c

···
 			       MSG_CMSG_COMPAT))
 		return -EOPNOTSUPP;
 
-	mutex_lock(&tls_ctx->tx_lock);
+	ret = mutex_lock_interruptible(&tls_ctx->tx_lock);
+	if (ret)
+		return ret;
 	lock_sock(sk);
 
 	if (unlikely(msg->msg_controllen)) {
···
 		      MSG_SENDPAGE_NOTLAST | MSG_SENDPAGE_NOPOLICY))
 		return -EOPNOTSUPP;
 
-	mutex_lock(&tls_ctx->tx_lock);
+	ret = mutex_lock_interruptible(&tls_ctx->tx_lock);
+	if (ret)
+		return ret;
 	lock_sock(sk);
 	ret = tls_sw_do_sendpage(sk, page, offset, size, flags);
 	release_sock(sk);
···
 
 	if (!test_and_clear_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask))
 		return;
-	mutex_lock(&tls_ctx->tx_lock);
-	lock_sock(sk);
-	tls_tx_records(sk, -1);
-	release_sock(sk);
-	mutex_unlock(&tls_ctx->tx_lock);
+
+	if (mutex_trylock(&tls_ctx->tx_lock)) {
+		lock_sock(sk);
+		tls_tx_records(sk, -1);
+		release_sock(sk);
+		mutex_unlock(&tls_ctx->tx_lock);
+	} else if (!test_and_set_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask)) {
+		/* Someone is holding the tx_lock, they will likely run Tx
+		 * and cancel the work on their way out of the lock section.
+		 * Schedule a long delay just in case.
+		 */
+		schedule_delayed_work(&ctx->tx_work.work, msecs_to_jiffies(10));
+	}
 }
 
 static bool tls_is_tx_ready(struct tls_sw_context_tx *ctx)