Re: [PATCH net] tun: free page on short-frame rejection in tun_xdp_one()

[Willem de Bruijn]

Weiming Shi wrote:
> tun_xdp_one() returns -EINVAL on a frame shorter than ETH_HLEN without
> freeing the page that vhost_net_build_xdp() allocated for it.
> tun_sendmsg() discards that -EINVAL and still returns total_len, so
> vhost_tx_batch() takes the success path and never frees the page; each
> short frame in a batch leaks one page-frag chunk.
> 
> A local process that can open /dev/net/tun and /dev/vhost-net can hit
> this path: it attaches a tun/tap device as the vhost-net backend and
> feeds TX descriptors whose length minus the virtio-net header is below
> ETH_HLEN. Each kick leaks the page-frag chunks for that batch, and a
> tight submission loop exhausts host memory and triggers an OOM panic.
> Free the page before returning -EINVAL, matching the XDP-program error
> path in the same function.
> 
> Fixes: 049584807f1d ("tun: add missing verification for short frame")
> Reported-by: Xiang Mei <xmei5@xxxxxxx>
> Assisted-by: Claude:claude-opus-4-7
> Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>
> ---
>  drivers/net/tun.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index b183189f1853..f594360d66d6 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -2394,8 +2394,10 @@ static int tun_xdp_one(struct tun_struct *tun,
>  	bool skb_xdp = false;
>  	struct page *page;
>  
> -	if (unlikely(datasize < ETH_HLEN))
> +	if (unlikely(datasize < ETH_HLEN)) {
> +		put_page(virt_to_head_page(xdp->data));
>  		return -EINVAL;
> +	}

Make sense, thanks.

The error path from tun_xdp_act does the same. And the default: label
used to too, before a batching optimization was introduced.

Is the same then also missing if build_skb fails?