Subject: Re: [PATCH net v3] net: gro: don't merge zcopy skbs

From: Willem de Bruijn <[email-masked]>
Date: Wed, 20 May 2026 17:38:31 -0400

Sabrina Dubroca wrote:
> skb_gro_receive() can currently copy frags between the source and GRO
> skb, without checking the zerocopy status, and in particular the
> SKBFL_MANAGED_FRAG_REFS flag.
> 
> When SKBFL_MANAGED_FRAG_REFS is set, the skb doesn't hold a reference
> on the pages in shinfo->frags. Appending those frags to another skb's
> frags without fixing up the page refcount can lead to UAF.
> 
> When either the last skb in the GRO chain (the one we would append
> frags to) or the source skb is zerocopy, don't merge the skbs.
> 
> Fixes: 753f1ca4e1e5 ("net: introduce managed frags infrastructure")
> Reported-by: Huzaifa Sidhpurwala <[email-masked]>
> Assisted-by: Claude:claude-mythos-preview
> Signed-off-by: Sabrina Dubroca <[email-masked]>
> ---
>  net/core/gro.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> Huzaifa has found this to be exploitable to overwrite the page cache
> 
> v3: maybe no stupid mistake this time. sorry again.
> v2: as Eric suggested, don't merge those skbs
>     https://lore.kernel.org/netdev/[email-masked]/
> v1: https://lore.kernel.org/netdev/[email-masked]/
> 
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 31d21de5b15a..bae89062a053 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -109,6 +109,9 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>  	if (p->pp_recycle != skb->pp_recycle)
>  		return -ETOOMANYREFS;
>  
> +	if (skb_zcopy(p) || skb_zcopy(skb))
> +		return -ETOOMANYREFS;
> +

As Pavel pointed out in a previous version, MSG_ZEROCOPY skbs are not
looped onto the receive path. From that PoV it's fine to add this
check as it will not match them.

What is the path that leads to managed frag skbs in the receive path?
I can believe that it exists, e.g., a page pool. But since Pavel also
brought up this question and knows the managed frag use much better
than I do.

>  	if (unlikely(p->len + len >= netif_get_gro_max_size(p->dev, p) ||
>  		     NAPI_GRO_CB(skb)->flush))
>  		return -E2BIG;
> -- 
> 2.54.0
>