[PATCH net 2/4] rxrpc: Fix conn-level packet handling to unshare RESPONSE packets

[David Howells <dhowells@redhat.com>]

The security operations that verify the RESPONSE packets decrypt bits of it
in place - however, the sk_buff may be shared with a packet sniffer, which
would lead to the sniffer seeing an apparently corrupt packet (actually
decrypted).

Fix this by unsharing the skbuff before handing it off to the specific
security handler.

Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Closes: https://sashiko.dev/#/patchset/20260408121252.2249051-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@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
---
 net/rxrpc/ar-internal.h |  2 +-
 net/rxrpc/conn_event.c  | 12 ++++++++++--
 net/rxrpc/io_thread.c   | 15 +++------------
 net/rxrpc/skbuff.c      | 26 ++++++++++++++++++++++----
 4 files changed, 36 insertions(+), 19 deletions(-)

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 96ecb83c9071..fb04d2ffdb27 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -1486,7 +1486,7 @@ int rxrpc_server_keyring(struct rxrpc_sock *, sockptr_t, int);
 void rxrpc_kernel_data_consumed(struct rxrpc_call *, struct sk_buff *);
 void rxrpc_new_skb(struct sk_buff *, enum rxrpc_skb_trace);
 void rxrpc_see_skb(struct sk_buff *, enum rxrpc_skb_trace);
-void rxrpc_eaten_skb(struct sk_buff *, enum rxrpc_skb_trace);
+struct sk_buff *rxrpc_unshare_skb(struct sk_buff **_old, gfp_t gfp);
 void rxrpc_get_skb(struct sk_buff *, enum rxrpc_skb_trace);
 void rxrpc_free_skb(struct sk_buff *, enum rxrpc_skb_trace);
 void rxrpc_purge_queue(struct sk_buff_head *);
diff --git a/net/rxrpc/conn_event.c b/net/rxrpc/conn_event.c
index 9a41ec708aeb..3d56a5d23369 100644
--- a/net/rxrpc/conn_event.c
+++ b/net/rxrpc/conn_event.c
@@ -244,8 +244,9 @@ static void rxrpc_call_is_secure(struct rxrpc_call *call)
  * connection-level Rx packet processor
  */
 static int rxrpc_process_event(struct rxrpc_connection *conn,
-			       struct sk_buff *skb)
+			       struct sk_buff **_skb)
 {
+	struct sk_buff *skb = *_skb;
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	bool secured = false;
 	int ret;
@@ -270,6 +271,13 @@ static int rxrpc_process_event(struct rxrpc_connection *conn,
 		}
 		spin_unlock_irq(&conn->state_lock);
 
+		skb = rxrpc_unshare_skb(_skb, GFP_NOFS);
+		if (!skb)
+			return -ENOMEM;
+
+		/* If unshared, skb will have changed. */
+		sp = rxrpc_skb(skb);
+
 		ret = conn->security->verify_response(conn, skb);
 		if (ret < 0)
 			return ret;
@@ -371,7 +379,7 @@ static void rxrpc_do_process_connection(struct rxrpc_connection *conn)
 	 * connection that each one has when we've finished with it */
 	while ((skb = skb_dequeue(&conn->rx_queue))) {
 		rxrpc_see_skb(skb, rxrpc_skb_see_conn_work);
-		ret = rxrpc_process_event(conn, skb);
+		ret = rxrpc_process_event(conn, &skb);
 		switch (ret) {
 		case -ENOMEM:
 		case -EAGAIN:
diff --git a/net/rxrpc/io_thread.c b/net/rxrpc/io_thread.c
index 697956931925..0592ce644fc3 100644
--- a/net/rxrpc/io_thread.c
+++ b/net/rxrpc/io_thread.c
@@ -249,19 +249,10 @@ static bool rxrpc_input_packet(struct rxrpc_local *local, struct sk_buff **_skb)
 		 * decryption.
 		 */
 		if (sp->hdr.securityIndex != 0) {
-			skb = skb_unshare(skb, GFP_ATOMIC);
-			if (!skb) {
-				rxrpc_eaten_skb(*_skb, rxrpc_skb_eaten_by_unshare_nomem);
-				*_skb = NULL;
+			skb = rxrpc_unshare_skb(_skb, GFP_ATOMIC);
+			if (!skb)
 				return just_discard;
-			}
-
-			if (skb != *_skb) {
-				rxrpc_eaten_skb(*_skb, rxrpc_skb_eaten_by_unshare);
-				*_skb = skb;
-				rxrpc_new_skb(skb, rxrpc_skb_new_unshared);
-				sp = rxrpc_skb(skb);
-			}
+			sp = rxrpc_skb(skb);
 		}
 		break;
 
diff --git a/net/rxrpc/skbuff.c b/net/rxrpc/skbuff.c
index 3bcd6ee80396..0dca9ca163f1 100644
--- a/net/rxrpc/skbuff.c
+++ b/net/rxrpc/skbuff.c
@@ -47,12 +47,30 @@ void rxrpc_get_skb(struct sk_buff *skb, enum rxrpc_skb_trace why)
 }
 
 /*
- * Note the dropping of a ref on a socket buffer by the core.
+ * Do the unsharing of a socket buffer, noting the event in the traces.
  */
-void rxrpc_eaten_skb(struct sk_buff *skb, enum rxrpc_skb_trace why)
+struct sk_buff *rxrpc_unshare_skb(struct sk_buff **_old, gfp_t gfp)
 {
-	int n = atomic_inc_return(&rxrpc_n_rx_skbs);
-	trace_rxrpc_skb(skb, 0, n, why);
+	struct sk_buff *skb, *old = *_old;
+	int n, r = refcount_read(&old->users);
+
+	skb = skb_unshare(old, gfp);
+	if (!skb) {
+		n = atomic_dec_return(&rxrpc_n_rx_skbs);
+		trace_rxrpc_skb(old, r, n, rxrpc_skb_eaten_by_unshare_nomem);
+		*_old = NULL;
+		return skb;
+	}
+
+	if (skb != old) {
+		n = atomic_read(&rxrpc_n_rx_skbs);
+		trace_rxrpc_skb(old, r, n, rxrpc_skb_eaten_by_unshare);
+		trace_rxrpc_skb(skb, refcount_read(&skb->users), n,
+				rxrpc_skb_new_unshared);
+		*_old = skb;
+	}
+
+	return skb;
 }
 
 /*