Re: [PATCH net] net: ax25: fix integer overflow in ax25_rx_fragment()

Joerg Reuter
2026-04-08T21:31:31Z

Am Thu, Apr 09, 2026 at 01:25:21AM +0800 schrieb Mashiro Chen:
> An attacker on an AX.25 link that supports multi-fragment I-frames
> (AX25_SEG_FIRST / AX25_SEG_REM mechanism) can trigger this by
> sending enough continuation fragments to wrap the 16-bit counter.
> With AX.25 segment numbers limited to 6 bits (max 63 continuation
> fragments), a fragment payload of ~1040 bytes per fragment is
> sufficient to overflow.

Even worse, it's 7 bits: https://www.ax25.net/AX25.2.2-Jul%2098-2.pdf
Figure 6.2 "Segment Header Format". Sigh.

Thanks,
Joerg

Acked-by: Joerg Reuter <jreuter@yaina.de>
> Cc: stable@vger.kernel.org
> Cc: linux-hams@vger.kernel.org
> Signed-off-by: Mashiro Chen <mashiro.chen@mailbox.org>
> ---
> net/ax25/ax25_in.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
> index d75b3e9ed93de8..68202c19b19e3f 100644
> --- a/net/ax25/ax25_in.c
> +++ b/net/ax25/ax25_in.c
> @@ -41,6 +41,11 @@ static int ax25_rx_fragment(ax25_cb *ax25, struct sk_buff *skb)
> /* Enqueue fragment */
> ax25->fragno = *skb->data & AX25_SEG_REM;
> skb_pull(skb, 1); /* skip fragno */
> + if ((unsigned int)ax25->fraglen + skb->len > USHRT_MAX) {
> + skb_queue_purge(&ax25->frag_queue);
> + ax25->fragno = 0;
> + return 1;
> + }
> ax25->fraglen += skb->len;
> skb_queue_tail(&ax25->frag_queue, skb);
>
> --
> 2.53.0
>

--
Joerg Reuter http://yaina.de/jreuter
And I make my way to where the warm scent of soil fills the evening air.
Everything is waiting quietly out there.... (Anne Clark)