Subject: Re: [PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

From: David Laight <[email-masked]>
Date: Tue, 21 Apr 2026 09:41:56 +0100

On Tue, 21 Apr 2026 05:46:26 +0000
Ashutosh Desai <[email-masked]> wrote:

> A crafted AX.25 frame with a valid address header but no control byte
> causes skb->len to reach zero after skb_pull() strips the header.
> The subsequent reads of skb->data[0] (control) and skb->data[1] (PID)
> are then out of bounds.
> 
> Linearize the skb after confirming the device is an AX.25 interface.
> Guard with skb->len < 1 after the pull - one byte suffices for LAPB
> control frames which have no PID byte. Add a separate skb->len < 2
> check inside the UI branch before accessing the PID byte.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: [email-masked]
> Signed-off-by: Ashutosh Desai <[email-masked]>
> ---
> v5:
> - Move skb_linearize() to after ax25_dev_ax25dev() check; avoids
>   unnecessary allocation for frames on non-AX.25 interfaces

Nitpick: 'on interfaces where AX.25 isn't enabled'
They still have to be AX.25 frames and get discarded.
So they won't really be expected and any allocated memory is
immediately freed.
More relevant would be linearizing before the ax25_addr_parse() call.
In any case I suspect this code never sees non-linear packets.
The packets will all be short, I don't know ax25, but X.25 (which I've
implemented most of in the past) originally had an mtu of 128 bytes
(and real links running at 2400 baud).

	David

> - Lower general guard from skb->len < 2 to skb->len < 1; the stricter
>   limit incorrectly dropped valid 1-byte LAPB control frames (SABM,
>   DISC, UA, DM, RR) which carry no PID byte
> - Add explicit skb->len < 2 check inside UI branch before the PID
>   byte (skb->data[1]) access
> v4:
> - Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
>   skb->len < 2 check (per David Laight review)
> v3:
> - Remove incorrect Suggested-by; add Fixes:, Cc: stable@
> v2:
> - Replace skb->len check with pskb_may_pull(skb, 2)
> 
> Link to v4: https://lore.kernel.org/netdev/[email-masked]/
> Link to v3: https://lore.kernel.org/netdev/[email-masked]/
> Link to v2: https://lore.kernel.org/netdev/[email-masked]/
> Link to v1: https://lore.kernel.org/netdev/[email-masked]/
> 
>  net/ax25/ax25_in.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
> index d75b3e9ed93d..c81d6830af48 100644
> --- a/net/ax25/ax25_in.c
> +++ b/net/ax25/ax25_in.c
> @@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
>  		goto free;
>  
> +	if (skb_linearize(skb))
> +		goto free;
> +
>  	/*
>  	 *	Parse the address header.
>  	 */
> @@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  	 */
>  	skb_pull(skb, ax25_addr_size(&dp));
>  
> +	if (skb->len < 1)
> +		goto free;
> +
>  	/* For our port addresses ? */
>  	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
>  		mine = 1;
> @@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  
>  	/* UI frame - bypass LAPB processing */
>  	if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) {
> +		if (skb->len < 2)
> +			goto free;
> +
>  		skb_set_transport_header(skb, 2); /* skip control and pid */
>  
>  		ax25_send_to_raw(&dest, skb, skb->data[1]);