Re: [PATCH net] net: skbuff: propagate shared-frag marker through pskb_copy()

[Ben Hutchings <ben@decadent.org.uk>]

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On Wed, 2026-05-13 at 20:25 +0900, Hyunwoo Kim wrote:
> __pskb_copy_fclone() shallow-copies the source's frag descriptors and
> bumps each page's refcount via skb_frag_ref(), then defers the rest
> of the shinfo metadata to skb_copy_header().  That helper only carries
> over gso_{size,segs,type} and never touches skb_shinfo()->flags, so
> the destination skb keeps a reference to the same externally-owned or
> page-cache-backed pages while reporting skb_has_shared_frag() as
> false.
>
> The mismatch is harmful in any in-place writer that uses
> skb_has_shared_frag() to decide whether shared pages must be detoured
> through skb_cow_data().  ESP input is one such writer (esp4.c,
> esp6.c), and a single nft 'dup to <local>' rule -- or any other
> nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d
> skb in esp_input() with the marker stripped, letting an unprivileged
> user write into the page cache of a root-owned read-only file via
> authencesn-ESN stray writes.
> 
> Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors
> were actually moved from the source.  skb_copy() and skb_copy_expand()
> share skb_copy_header() too but linearize all paged data into freshly
> allocated head storage and emerge with nr_frags == 0, so
> skb_has_shared_frag() returns false on its own; they need no change.

What about skb_shift()?  It seems like that should also propagate this
flag.  But I could be missing some reason why it's not necessary.

Ben.

> Fixes: cef401de7be8 ("net: fix possible wrong checksum generation")
> Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
> Reported-by: William Bowling <vakzz@zellic.io>
> Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> ---
>  net/core/skbuff.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7dad68e3b518..15bdec53e8d9 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2248,6 +2248,7 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom,
>  			skb_frag_ref(skb, i);
>  		}
>  		skb_shinfo(n)->nr_frags = i;
> +		skb_shinfo(n)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
>  	}
>  
>  	if (skb_has_frag_list(skb)) {
> @@ -6200,6 +6201,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  	       from_shinfo->frags,
>  	       from_shinfo->nr_frags * sizeof(skb_frag_t));
>  	to_shinfo->nr_frags += from_shinfo->nr_frags;
> +	if (from_shinfo->nr_frags)
> +		to_shinfo->flags |= from_shinfo->flags & SKBFL_SHARED_FRAG;
>  
>  	if (!skb_cloned(from))
>  		from_shinfo->nr_frags = 0;

-- 
Ben Hutchings
Tomorrow will be cancelled due to lack of interest.

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