[PATCH net] net/tls: Consume empty data records in tls_sw_read_sock()

[Chuck Lever <cel@kernel.org>]

A peer may send a zero-length TLS application_data record; TLS 1.3
explicitly permits these as a traffic-analysis countermeasure (RFC
8446, Section 5.1). After decryption such a record has full_len ==
0. tls_sw_read_sock() hands it to the read_actor, which has no
payload to consume and returns zero. The loop treats a zero return
as backpressure (used <= 0), requeues the skb at the head of
rx_list, and stops. rx_list is serviced head-first on the next
call, so the empty record is dequeued, fails the same way, and is
requeued again; every later record on the connection is blocked
behind it.

tls_sw_recvmsg() does not stall on this: a zero-length data record
copies nothing and falls through to consume_skb(). Mirror that in
the read_sock() path by recognizing an empty data record before
the actor runs, consuming it, and continuing.

Fixes: 662fbcec32f4 ("net/tls: implement ->read_sock()")
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 net/tls/tls_sw.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 9324e4ed20a3..d4afc90fd796 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -2115,6 +2115,17 @@ int tls_sw_read_sock(struct sock *sk, read_descriptor_t *desc,
 			goto read_sock_requeue;
 		}
 
+		/* An empty data record (legal in TLS 1.3) gives a zero
+		 * read_actor return, indistinguishable from the consumer
+		 * stalling; the used <= 0 path would requeue it at the
+		 * head of rx_list and block all later records. Consume it
+		 * here instead.
+		 */
+		if (rxm->full_len == 0) {
+			consume_skb(skb);
+			continue;
+		}
+
 		used = read_actor(desc, skb, rxm->offset, rxm->full_len);
 		if (used <= 0) {
 			if (!copied)
-- 
2.54.0