Re: [PATCH v2] xfrm: fix missing headroom check in xfrm_dev_direct_output

[Steffen Klassert <steffen.klassert@secunet.com>]

On Mon, May 18, 2026 at 10:04:09AM +0200, Günther Muller wrote:
> Ensure the skb has enough headroom for the hardware offload device's
> hard_header_len. If the headroom is insufficient (e.g., when routing
> through certain virtual or tunnel devices), __skb_push() underflows
> skb->data below skb->head, causing silent memory corruption.
> 
> Fix this by using skb_cow_head() to dynamically expand headroom if
> needed, and switch to the checked skb_push() variant.
> 
> Fixes: 5eddd76ec2fd ("xfrm: fix tunnel mode TX datapath in packet offload mode")
> 
> Signed-off-by: Günther Muller <gunther.muller2008@gmail.com>
> ---
>  net/xfrm/xfrm_output.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
> index cc35c2fcbb..35f974d209 100644
> --- a/net/xfrm/xfrm_output.c
> +++ b/net/xfrm/xfrm_output.c
> @@ -649,7 +649,10 @@ static int xfrm_dev_direct_output(struct sock *sk, struct xfrm_state *x,
>  	 * to netdevice.
>  	 */
>  	skb->dev = x->xso.dev;
> -	__skb_push(skb, skb->dev->hard_header_len);
> +	err = skb_cow_head(skb, skb->dev->hard_header_len);
> +	if (err)
> +		return err;

You leak skb here. Also, we should bump a statistic counter
when droping the packet.