[PATCH 6.18 242/275] rxgk: Fix potential integer overflow in length check

[Greg Kroah-Hartman]

6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells at redhat.com>

commit 6929350080f4da292d111a3b33e53138fee51cec upstream.

Fix potential integer overflow in rxgk_extract_token() when checking the
length of the ticket.  Rather than rounding up the value to be tested
(which might overflow), round down the size of the available data.

Fixes: 2429a1976481 ("rxrpc: Fix untrusted unsigned subtract")
Closes: https://sashiko.dev/#/patchset/20260408121252.2249051-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Marc Dionne <marc.dionne at auristor.com>
cc: Jeffrey Altman <jaltman at auristor.com>
cc: Simon Horman <horms at kernel.org>
cc: linux-afs at lists.infradead.org
cc: stable at kernel.org
Link: https://patch.msgid.link/20260422161438.2593376-6-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
---
 net/rxrpc/rxgk_app.c    | 2 +-
 net/rxrpc/rxgk_common.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/rxrpc/rxgk_app.c b/net/rxrpc/rxgk_app.c
index 30275cb5ba3e..5587639d60c5 100644
--- a/net/rxrpc/rxgk_app.c
+++ b/net/rxrpc/rxgk_app.c
@@ -214,7 +214,7 @@ int rxgk_extract_token(struct rxrpc_connection *conn, struct sk_buff *skb,
 	ticket_len	= ntohl(container.token_len);
 	ticket_offset	= token_offset + sizeof(container);
 
-	if (xdr_round_up(ticket_len) > token_len - sizeof(container))
+	if (ticket_len > xdr_round_down(token_len - sizeof(container)))
 		goto short_packet;
 
 	_debug("KVNO %u", kvno);
diff --git a/net/rxrpc/rxgk_common.h b/net/rxrpc/rxgk_common.h
index 80164d89e19c..1e257d7ab8ec 100644
--- a/net/rxrpc/rxgk_common.h
+++ b/net/rxrpc/rxgk_common.h
@@ -34,6 +34,7 @@ struct rxgk_context {
 };
 
 #define xdr_round_up(x) (round_up((x), sizeof(__be32)))
+#define xdr_round_down(x) (round_down((x), sizeof(__be32)))
 #define xdr_object_len(x) (4 + xdr_round_up(x))
 
 /*
-- 
2.54.0