Subject: Re: [PATCH net v4 2/5] xsk: fix buffer leak in xsk_drop_skb() for AF_XDP multi-buffer Tx

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 21 May 2026 14:05:39 +0200

On Wed, May 20, 2026 at 08:42:41AM +0800, Jason Xing wrote:
> From: Jason Xing <[email-masked]>
> 
> This patch is inspired by the check[1] from sashiko. It says when
> overflow happens, the address of cq to be published is invalid.
> Actually the severer thing is the whole process of publishing the
> address of cq in this particular case is not right: it should truely
> publish the address and advance the cached_prod in cq as long as it
> reads descriptors from txq.
> 
> The following is the full analysis.
> xsk_drop_skb() is called in three places, which all discard a partially
> built multi-buffer skb:
> 1) xsk_build_skb() -EOVERFLOW error path: packet exceeds MAX_SKB_FRAGS
> 2) __xsk_generic_xmit() post-loop cleanup: an invalid descriptor in
>    the TX ring prevents the partial packet from completing
> 3) xsk_release(): socket close while xs->skb holds an incomplete packet
> 
> In all three cases, the TX descriptors for the already-processed frags
> have been consumed from the TX ring (xskq_cons_release), and CQ slots
> have been reserved. However, xsk_drop_skb() calls xsk_consume_skb()
> which cancels the CQ reservations via xsk_cq_cancel_locked(). Since
> the buffer addresses never appear in the completion queue, userspace
> permanently loses track of these buffers.
> 
> Fix this by letting consume_skb() trigger the existing xsk_destruct_skb
> destructor, which already submits buffer addresses to the CQ via
> xsk_cq_submit_addr_locked().
> 
> Note that cancelling the descriptors back to the TX ring (via
> xskq_cons_cancel_n) is not a appropriate option because an oversized
> packet that always exceeds MAX_SKB_FRAGS would be retried indefinitely,
> which is an obviously deadlock bug in the TX path.
> 
> Also move the desc->addr assignment in xsk_build_skb() above the
> overflow check so that the current descriptor's address is recorded
> before a potential -EOVERFLOW jump to free_err, consistent with the
> zerocopy path in xsk_build_skb_zerocopy().
> 
> [1]: https://lore.kernel.org/all/[email-masked]/
> 
> Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
> Signed-off-by: Jason Xing <[email-masked]>

Sorry for the noise, got lost in my inbox and replied on v3.

Reviewed-by: Maciej Fijalkowski <[email-masked]>

> ---
>  net/xdp/xsk.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f8c8a8c9dfba..0a6203c42576 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -793,8 +793,11 @@ static void xsk_consume_skb(struct sk_buff *skb)
>  
>  static void xsk_drop_skb(struct sk_buff *skb)
>  {
> -	xdp_sk(skb->sk)->tx->invalid_descs += xsk_get_num_desc(skb);
> -	xsk_consume_skb(skb);
> +	struct xdp_sock *xs = xdp_sk(skb->sk);
> +
> +	xs->tx->invalid_descs += xsk_get_num_desc(skb);
> +	consume_skb(skb);
> +	xs->skb = NULL;
>  }
>  
>  static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
> @@ -876,7 +879,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>  			return ERR_PTR(-ENOMEM);
>  
>  		/* in case of -EOVERFLOW that could happen below,
> -		 * xsk_consume_skb() will release this node as whole skb
> +		 * xsk_drop_skb() will release this node as whole skb
>  		 * would be dropped, which implies freeing all list elements
>  		 */
>  		xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
> @@ -968,6 +971,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>  				goto free_err;
>  			}
>  
> +			xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
> +
>  			if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) {
>  				err = -EOVERFLOW;
>  				goto free_err;
> @@ -985,8 +990,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>  
>  			skb_add_rx_frag(skb, nr_frags, page, 0, len, PAGE_SIZE);
>  			refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc);
> -
> -			xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
>  		}
>  	}
>  
> -- 
> 2.43.7
>