In rxrpc_preparse(), there are two paths for parsing key payloads: the
XDR path (for large payloads) and the non-XDR path (for payloads <= 28
bytes). While the XDR path (rxrpc_preparse_xdr_rxkad()) correctly
validates the ticket length against AFSTOKEN_RK_TIX_MAX, the non-XDR
path fails to do so.

This allows an unprivileged user to provide a very large ticket length.
When this key is later read via rxrpc_read(), the total
token size (toksize) calculation results in a value that exceeds
AFSTOKEN_LENGTH_MAX, triggering a WARN_ON().

[ 2001.302904] WARNING: CPU: 2 PID: 2108 at net/rxrpc/key.c:778 rxrpc_read+0x109/0x5c0 [rxrpc]

Fix this by adding a check in the non-XDR parsing path of rxrpc_preparse()
to ensure the ticket length does not exceed AFSTOKEN_RK_TIX_MAX,
bringing it into parity with the XDR parsing logic.

Fixes: 8a7a3eb4ddbe ("KEYS: RxRPC: Use key preparsing")
Fixes: 84924aac08a4 ("rxrpc: Fix checker warning")
Reported-by: Anderson Nascimento <anderson@allelesecurity.com>
Signed-off-by: Anderson Nascimento <anderson@allelesecurity.com>
---
 net/rxrpc/key.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/rxrpc/key.c b/net/rxrpc/key.c
index 6301d79ee35a..5ebb06d87cdd 100644
--- a/net/rxrpc/key.c
+++ b/net/rxrpc/key.c
@@ -502,6 +502,10 @@ static int rxrpc_preparse(struct key_preparsed_payload *prep)
 	if (v1->security_index != RXRPC_SECURITY_RXKAD)
 		goto error;
 
+	ret = -EKEYREJECTED;
+	if(v1->ticket_length > AFSTOKEN_RK_TIX_MAX)
+		goto error;
+
 	plen = sizeof(*token->kad) + v1->ticket_length;
 	prep->quotalen += plen + sizeof(*token);
 
-- 
2.53.0

> Subject: [PATCH] rxrpc: fix missing validation of ticket length in

...in rxrpc_preparse(), I presume?

David

On Wed, Apr 22, 2026 at 9:38 AM David Howells <dhowells@redhat.com> wrote:
>
> > Subject: [PATCH] rxrpc: fix missing validation of ticket length in
>
> ...in rxrpc_preparse(), I presume?
>

In fact it was "non-XDR key preparsing", but "in rxrpc_preparse()" also works.

> David
>

-- 
Anderson Nascimento
Allele Security Intelligence
https://www.allelesecurity.com