Subject: Re: [PATCH net v4] net: skbuff: propagate shared-frag marker through frag-transfer helpers

From: Hyunwoo Kim <[email-masked]>
Date: Fri, 15 May 2026 15:07:37 +0900

On Fri, May 15, 2026 at 02:55:35PM +0900, Hyunwoo Kim wrote:
> Two frag-transfer helpers (__pskb_copy_fclone() and skb_shift()) fail
> to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when
> moving frags from source to destination.  __pskb_copy_fclone() defers
> the rest of the shinfo metadata to skb_copy_header() after copying
> frag descriptors, but that helper only carries over gso_{size,segs,
> type} and never touches skb_shinfo()->flags; skb_shift() moves frag
> descriptors directly and leaves flags untouched.  As a result, 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.
> 
> The same omission exists in skb_gro_receive() and skb_gro_receive_list().
> The former moves the incoming skb's frag descriptors into the
> accumulator's last sub-skb via two paths (a direct frag-move loop and
> the head_frag + memcpy path); the latter chains the incoming skb whole
> onto p's frag_list.  Downstream skb_segment() reads only
> skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's
> shinfo as the nskb -- both p and lp must carry the marker.
> 
> The same omission also exists in tcp_clone_payload(), which builds an
> MTU probe skb by moving frag descriptors from skbs on sk_write_queue
> into a freshly allocated nskb.  The helper falls into the same family
> and warrants the same fix for consistency; no TCP TX-side in-place
> writer is currently known to reach a user page through this gap, but
> a future consumer depending on the marker would regress silently.
> 
> Fixes: cef401de7be8 ("net: fix possible wrong checksum generation")
> Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
> Suggested-by: Sabrina Dubroca <[email-masked]>
> Suggested-by: Sultan Alsawaf <[email-masked]>
> Suggested-by: Ben Hutchings <[email-masked]>

Since they are asking for credit, I will add them:

Suggested-by: Lin Ma <[email-masked]>
Suggested-by: Jingguo Tan <[email-masked]>

> Cc: [email-masked]
> Signed-off-by: Hyunwoo Kim <[email-masked]>
> ---
> Changes in v4:
> - Include the tcp_clone_payload() propagation suggested by Sabrina.
> - Drop the skb_try_coalesce() change; addressed by commit f84eca581739.
> - v3: https://lore.kernel.org/all/agW4vC0r8QOUKtRT@v4bel/
> 
> Changes in v3:
> - Include the skb_gro_receive() audit patch suggested by Sultan
> - v2: https://lore.kernel.org/all/agToIEDI4TaTNLRb@v4bel/
> 
> Changes in v2:
> - Also propagate SHARED_FRAG in skb_try_coalesce() and skb_shift()
> - v1: https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/
> ---
>  net/core/gro.c        | 4 ++++
>  net/core/skbuff.c     | 3 +++
>  net/ipv4/tcp_output.c | 1 +
>  3 files changed, 8 insertions(+)
> 
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 31d21de5b15a..9f8960789b2c 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -213,10 +213,12 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>  	p->data_len += len;
>  	p->truesize += delta_truesize;
>  	p->len += len;
> +	skb_shinfo(p)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
>  	if (lp != p) {
>  		lp->data_len += len;
>  		lp->truesize += delta_truesize;
>  		lp->len += len;
> +		skb_shinfo(lp)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
>  	}
>  	NAPI_GRO_CB(skb)->same_flow = 1;
>  	return 0;
> @@ -244,6 +246,8 @@ int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
>  	p->truesize += skb->truesize;
>  	p->len += skb->len;
>  
> +	skb_shinfo(p)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
> +
>  	NAPI_GRO_CB(skb)->same_flow = 1;
>  
>  	return 0;
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 9c4e8d331d6d..7cd388504297 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)) {
> @@ -4349,6 +4350,8 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen)
>  	tgt->ip_summed = CHECKSUM_PARTIAL;
>  	skb->ip_summed = CHECKSUM_PARTIAL;
>  
> +	skb_shinfo(tgt)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
> +
>  	skb_len_add(skb, -shiftlen);
>  	skb_len_add(tgt, shiftlen);
>  
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f9d8755705f7..6e4bb411dc04 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2626,6 +2626,7 @@ static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
>  			todo = min_t(int, skb_frag_size(fragfrom),
>  				     probe_size - len);
>  			len += todo;
> +			skb_shinfo(to)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
>  			if (lastfrag &&
>  			    skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
>  			    skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +
> -- 
> 2.43.0
>