From: Alok Tiwari <redacted>

In rxrpc_post_response(), the code should be comparing the challenge serial
number from the cached response before deciding to switch to a newer
response, but looks at the newer packet private data instead, rendering the
comparison always false.

Fix this by switching to look at the older packet.

Fix further[1] to substitute the new packet in place of the old one if
newer and also to release whichever we don't use.

Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Signed-off-by: Alok Tiwari <redacted>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <redacted>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org
Link: https://sashiko.dev/#/patchset/20260319150150.4189381-1-dhowells%40redhat.com [1]
---
 include/trace/events/rxrpc.h | 1 +
 net/rxrpc/conn_event.c       | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h
index 869f97c9bf73..5edad6a624ad 100644
--- a/include/trace/events/rxrpc.h
+++ b/include/trace/events/rxrpc.h

@@ -185,6 +185,7 @@
 	EM(rxrpc_skb_put_input,			"PUT input    ") \
 	EM(rxrpc_skb_put_jumbo_subpacket,	"PUT jumbo-sub") \
 	EM(rxrpc_skb_put_oob,			"PUT oob      ") \
+	EM(rxrpc_skb_put_old_response,		"PUT old-resp ") \
 	EM(rxrpc_skb_put_purge,			"PUT purge    ") \
 	EM(rxrpc_skb_put_purge_oob,		"PUT purge-oob") \
 	EM(rxrpc_skb_put_response,		"PUT response ") \

diff --git a/net/rxrpc/conn_event.c b/net/rxrpc/conn_event.c
index 98ad9b51ca2c..c50cbfc5a313 100644
--- a/net/rxrpc/conn_event.c
+++ b/net/rxrpc/conn_event.c

@@ -557,11 +557,11 @@ void rxrpc_post_response(struct rxrpc_connection *conn, struct sk_buff *skb)
 	spin_lock_irq(&local->lock);
 	old = conn->tx_response;
 	if (old) {
-		struct rxrpc_skb_priv *osp = rxrpc_skb(skb);
+		struct rxrpc_skb_priv *osp = rxrpc_skb(old);
 
 		/* Always go with the response to the most recent challenge. */
 		if (after(sp->resp.challenge_serial, osp->resp.challenge_serial))
-			conn->tx_response = old;
+			conn->tx_response = skb;
 		else
 			old = skb;
 	} else {

@@ -569,4 +569,5 @@ void rxrpc_post_response(struct rxrpc_connection *conn, struct sk_buff *skb)
 	}
 	spin_unlock_irq(&local->lock);
 	rxrpc_poke_conn(conn, rxrpc_conn_get_poke_response);
+	rxrpc_free_skb(old, rxrpc_skb_put_old_response);
 }