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

From: Joerg Reuter <[email-masked]>
Date: Wed, 8 Apr 2026 23:31:27 +0200

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 <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Mashiro Chen <[email-masked]>
> ---
>  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)