[PATCH net v4 14/15] rxrpc: fix oversized RESPONSE authenticator length check

[David Howells]

From: Keenan Dong <keenanat2000 at gmail.com>

rxgk_verify_response() decodes auth_len from the packet and is supposed
to verify that it fits in the remaining bytes. The existing check is
inverted, so oversized RESPONSE authenticators are accepted and passed
to rxgk_decrypt_skb(), which can later reach skb_to_sgvec() with an
impossible length and hit BUG_ON(len).

Decoded from the original latest-net reproduction logs with
scripts/decode_stacktrace.sh:

RIP: __skb_to_sgvec()
  [net/core/skbuff.c:5285 (discriminator 1)]
Call Trace:
 skb_to_sgvec() [net/core/skbuff.c:5305]
 rxgk_decrypt_skb() [net/rxrpc/rxgk_common.h:81]
 rxgk_verify_response() [net/rxrpc/rxgk.c:1268]
 rxrpc_process_connection()
   [net/rxrpc/conn_event.c:266 net/rxrpc/conn_event.c:364
    net/rxrpc/conn_event.c:386]
 process_one_work() [kernel/workqueue.c:3281]
 worker_thread()
   [kernel/workqueue.c:3353 kernel/workqueue.c:3440]
 kthread() [kernel/kthread.c:436]
 ret_from_fork() [arch/x86/kernel/process.c:164]

Reject authenticator lengths that exceed the remaining packet payload.

Fixes: 9d1d2b59341f ("rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)")
Signed-off-by: Keenan Dong <keenanat2000 at gmail.com>
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Marc Dionne <marc.dionne at auristor.com>
cc: Eric Dumazet <edumazet at google.com>
cc: "David S. Miller" <davem at davemloft.net>
cc: Jakub Kicinski <kuba at kernel.org>
cc: Paolo Abeni <pabeni at redhat.com>
cc: Simon Horman <horms at kernel.org>
cc: Willy Tarreau <w at 1wt.eu>
cc: linux-afs at lists.infradead.org
cc: netdev at vger.kernel.org
cc: stable at kernel.org
---
 net/rxrpc/rxgk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/rxrpc/rxgk.c b/net/rxrpc/rxgk.c
index 01dbdf0b5cf2..9e4a4ff28913 100644
--- a/net/rxrpc/rxgk.c
+++ b/net/rxrpc/rxgk.c
@@ -1224,7 +1224,7 @@ static int rxgk_verify_response(struct rxrpc_connection *conn,
 
 	auth_offset	= offset;
 	auth_len	= ntohl(xauth_len);
-	if (auth_len < len)
+	if (auth_len > len)
 		goto short_packet;
 	if (auth_len & 3)
 		goto inconsistent;