[PATCH net v1] tls: fix hung task in tx_work_handler by using non-blocking sends

[Jiayuan Chen]

From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>

tx_work_handler calls tls_tx_records with flags=-1, which preserves
each record's original tx_flags but results in tcp_sendmsg_locked
using an infinite send timeout. When the peer is unresponsive and the
send buffer is full, tcp_sendmsg_locked blocks indefinitely in
sk_stream_wait_memory. This causes tls_sk_proto_close to hang in
cancel_delayed_work_sync waiting for tx_work_handler to finish,
leading to a hung task:

  INFO: task ...: blocked for more than ... seconds.
  Call Trace:
    cancel_delayed_work_sync
    tls_sw_cancel_work_tx
    tls_sk_proto_close

A workqueue handler should never block indefinitely. Fix this by
introducing __tls_tx_records() with an extra_flags parameter that
gets OR'd into each record's tx_flags. tx_work_handler uses this to
pass MSG_DONTWAIT so tcp_sendmsg_locked returns -EAGAIN immediately
when the send buffer is full, without overwriting the original
per-record flags (MSG_MORE, MSG_NOSIGNAL, etc.). On -EAGAIN, the
existing reschedule mechanism retries after a short delay.

Also consolidate the two identical reschedule paths (lock contention
and -EAGAIN) into one.

Reported-by: syzbot+ca1345cca66556f3d79b@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: a42055e8d2c3 ("net/tls: Add support for async encryption of records for performance")
Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
---
 net/tls/tls_sw.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 9937d4c810f2..c9d3d44581da 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -404,7 +404,7 @@ static void tls_free_open_rec(struct sock *sk)
 	}
 }
 
-int tls_tx_records(struct sock *sk, int flags)
+static int __tls_tx_records(struct sock *sk, int flags, int extra_flags)
 {
 	struct tls_context *tls_ctx = tls_get_ctx(sk);
 	struct tls_sw_context_tx *ctx = tls_sw_ctx_tx(tls_ctx);
@@ -417,9 +417,9 @@ int tls_tx_records(struct sock *sk, int flags)
 				       struct tls_rec, list);
 
 		if (flags == -1)
-			tx_flags = rec->tx_flags;
+			tx_flags = rec->tx_flags | extra_flags;
 		else
-			tx_flags = flags;
+			tx_flags = flags | extra_flags;
 
 		rc = tls_push_partial_record(sk, tls_ctx, tx_flags);
 		if (rc)
@@ -463,6 +463,11 @@ int tls_tx_records(struct sock *sk, int flags)
 	return rc;
 }
 
+int tls_tx_records(struct sock *sk, int flags)
+{
+	return __tls_tx_records(sk, flags, 0);
+}
+
 static void tls_encrypt_done(void *data, int err)
 {
 	struct tls_sw_context_tx *ctx;
@@ -2629,6 +2634,7 @@ static void tx_work_handler(struct work_struct *work)
 	struct sock *sk = tx_work->sk;
 	struct tls_context *tls_ctx = tls_get_ctx(sk);
 	struct tls_sw_context_tx *ctx;
+	int rc;
 
 	if (unlikely(!tls_ctx))
 		return;
@@ -2642,16 +2648,21 @@ static void tx_work_handler(struct work_struct *work)
 
 	if (mutex_trylock(&tls_ctx->tx_lock)) {
 		lock_sock(sk);
-		tls_tx_records(sk, -1);
+		rc = __tls_tx_records(sk, -1, MSG_DONTWAIT);
 		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));
+		if (rc != -EAGAIN)
+			return;
 	}
+
+	/* 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.
+	 * Also reschedule on -EAGAIN when the send buffer is full
+	 * to avoid blocking the workqueue indefinitely.
+	 */
+	if (!test_and_set_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask))
+		schedule_delayed_work(&ctx->tx_work.work, msecs_to_jiffies(10));
 }
 
 static bool tls_is_tx_ready(struct tls_sw_context_tx *ctx)
-- 
2.43.0