Re: [PATCH net v3 6/8] xsk: free the skb when hitting the upper bound MAX_SKB_FRAGS

[Stanislav Fomichev <sdf.kernel@gmail.com>]

> From: Jason Xing <kernelxing@tencent.com>
> 
> Fix it by explicitly adding kfree_skb() before returning back to its
> caller.
> 
> How to reproduce it in virtio_net:
> 1. the current skb is the first one (which means xs->skb is NULL) and
>    hit the limit MAX_SKB_FRAGS.
> 2. xsk_build_skb_zerocopy() returns -EOVERFLOW.
> 3. the caller xsk_build_skb() clears skb by using 'skb = NULL;'. This
>    is why bug can be triggered.
> 4. there is no chance to free this skb anymore.
> 
> Note that if in this case the xs->skb is not NULL, xsk_build_skb() will
> call xsk_drop_skb(xs->skb) to do the right thing.
> 
> Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>  net/xdp/xsk.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index c49b58199d2f..5e6326e076ab 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -776,8 +776,11 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>  	addr = buffer - pool->addrs;
>  
>  	for (copied = 0, i = skb_shinfo(skb)->nr_frags; copied < len; i++) {
> -		if (unlikely(i >= MAX_SKB_FRAGS))
> +		if (unlikely(i >= MAX_SKB_FRAGS)) {
> +			if (!xs->skb)
> +				kfree_skb(skb);
>  			return ERR_PTR(-EOVERFLOW);
> +		}
>  
>  		page = pool->umem->pgs[addr >> PAGE_SHIFT];
>  		get_page(page);
> --
> 2.41.3
>

Acked-by: Stanislav Fomichev <sdf@fomichev.me>