FAILED: patch "[PATCH] net: skbuff: propagate shared-frag marker through" failed to apply to 5.15-stable tree

PATCH In review [email protected] 3 participants 3 messages May 22 – May 23

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2026-05-22 07:39 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 48f6a5356a33dd78e7144ae1faef95ffc990aae0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026052230-kilogram-proving-cd58@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 48f6a5356a33dd78e7144ae1faef95ffc990aae0 Mon Sep 17 00:00:00 2001
From: Hyunwoo Kim <[email protected]>
Date: Sat, 16 May 2026 07:28:53 +0900
Subject: [PATCH] net: skbuff: propagate shared-frag marker through
frag-transfer helpers

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.

The same omission exists in skb_segment(): the per-iteration flag merge takes only head_skb's flag, and the inner switch that rebinds frag_skb to list_skb on head_skb-frags exhaustion does not fold the new frag_skb's flag into nskb. Fold frag_skb's flag at both sites so segments drawing frags from frag_list members carry the marker.

Fixes cef401de7be8 ("net: fix possible wrong checksum generation")

Fixes f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")

Suggested-by Lin Ma <[email protected]>

Cc [email protected]

Signed-off-by Hyunwoo Kim <[email protected]>

Tested-by Rajat Gupta <[email protected]>

Link https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel open here

Signed-off-by Paolo Abeni <[email protected]>

net/core/gro.c +4 -0

index 31d21de5b15a..9f8960789b2c 100644
@@ -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;

net/core/skbuff.c +8 -1

index 9c4e8d331d6d..44ac121cfccb 100644
@@ -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);

@@ -4959,7 +4962,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		skb_copy_from_linear_data_offset(head_skb, offset,
 						 skb_put(nskb, hsize), hsize);
 
-		skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
+		skb_shinfo(nskb)->flags |= (skb_shinfo(head_skb)->flags |
+					    skb_shinfo(frag_skb)->flags) &
 					   SKBFL_SHARED_FRAG;
 
 		if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))

@@ -4976,6 +4980,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 				nfrags = skb_shinfo(list_skb)->nr_frags;
 				frag = skb_shinfo(list_skb)->frags;
 				frag_skb = list_skb;
+
+				skb_shinfo(nskb)->flags |= skb_shinfo(frag_skb)->flags & SKBFL_SHARED_FRAG;
+
 				if (!skb_headlen(list_skb)) {
 					BUG_ON(!nfrags);
 				} else {

net/ipv4/tcp_output.c +1 -0

index f9d8755705f7..6e4bb411dc04 100644
@@ -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) +

Ben Hutchings

2026-05-22 19:46 UTC #1 lore raw

PATCH 1/1 net: skbuff: propagate shared-frag marker through frag-transfer helpers

From: Hyunwoo Kim <[email protected]>

commit 48f6a5356a33dd78e7144ae1faef95ffc990aae0 upstream.

Fixes cef401de7be8 ("net: fix possible wrong checksum generation")

Fixes f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")

Suggested-by Lin Ma <[email protected]>

Cc [email protected]

Signed-off-by Hyunwoo Kim <[email protected]>

Tested-by Rajat Gupta <[email protected]>

Link https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel open here

Signed-off-by Paolo Abeni <[email protected]>

[bwh: Backported to 5.15:
- skb_gro_receive() and skb_gro_receive_list() are in skbuff.c here
- Drop change to tcp_clone_payload(), which does not exist here
- Adjust context in skb_shift()
]

Signed-off-by Ben Hutchings <[email protected]>

---
 net/core/skbuff.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

net/core/skbuff.c +12 -1

index aadb87aa5e7e..a8d09eff26f1 100644
@@ -1661,6 +1661,7 @@ 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)) {

@@ -3650,6 +3651,8 @@ 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;
+
 	/* Yak, is it really working this way? Some helper please? */
 	skb->len -= shiftlen;
 	skb->data_len -= shiftlen;

@@ -4017,6 +4020,8 @@ 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;

@@ -4251,7 +4256,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		skb_copy_from_linear_data_offset(head_skb, offset,
 						 skb_put(nskb, hsize), hsize);
 
-		skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
+		skb_shinfo(nskb)->flags |= (skb_shinfo(head_skb)->flags |
+					    skb_shinfo(frag_skb)->flags) &
 					   SKBFL_SHARED_FRAG;
 
 		if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))

@@ -4268,6 +4274,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 				nfrags = skb_shinfo(list_skb)->nr_frags;
 				frag = skb_shinfo(list_skb)->frags;
 				frag_skb = list_skb;
+
+				skb_shinfo(nskb)->flags |= skb_shinfo(frag_skb)->flags & SKBFL_SHARED_FRAG;
+
 				if (!skb_headlen(list_skb)) {
 					BUG_ON(!nfrags);
 				} else {

@@ -4490,10 +4499,12 @@ 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;

Harshit Mogalapalli

Reviewed-by on v1 1/1

2026-05-23 06:05 UTC #2 lore raw

Re: [PATCH 5.15] net: skbuff: propagate shared-frag marker through frag-transfer helpers

Quoted 64 lines · Ben Hutchings

From: Hyunwoo Kim <[email protected]>
commit 48f6a5356a33dd78e7144ae1faef95ffc990aae0 upstream.
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.
The same omission exists in skb_segment(): the per-iteration flag merge takes only head_skb's flag, and the inner switch that rebinds frag_skb to list_skb on head_skb-frags exhaustion does not fold the new frag_skb's flag into nskb. Fold frag_skb's flag at both sites so segments drawing frags from frag_list members carry the marker.
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 protected]>
Suggested-by Sultan Alsawaf <[email protected]>
Suggested-by Ben Hutchings <[email protected]>
Suggested-by Lin Ma <[email protected]>
Suggested-by Jingguo Tan <[email protected]>
Suggested-by Aaron Esau <[email protected]>
Cc [email protected]
Signed-off-by Hyunwoo Kim <[email protected]>
Tested-by Rajat Gupta <[email protected]>
Link https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel open here
Signed-off-by Paolo Abeni <[email protected]>

Quoted 2 lines

[bwh: Backported to 5.15:
- skb_gro_receive() and skb_gro_receive_list() are in skbuff.c here

right

Quoted 1 line

- Drop change to tcp_clone_payload(), which does not exist here

right as we don't have commit: 736013292e3c ("tcp: let tcp_mtu_probe() build headless packets")

Quoted 2 lines

- Adjust context in skb_shift()
]

Right, looks good, as we don't have commit: ede57d58e6f3
("net: helper function skb_len_add") in 5.15.y

LGTM.

So from a backport point of view.

Reviewed-by Harshit Mogalapalli <[email protected]>

thanks,
Harshit

Quoted 68 lines

Signed-off-by Ben Hutchings <[email protected]>

---
  net/core/skbuff.c | 13 ++++++++++++-
  1 file changed, 12 insertions(+), 1 deletion(-)

net/core/skbuff.c +12 -1

index aadb87aa5e7e..a8d09eff26f1 100644
@@ -1661,6 +1661,7 @@ 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)) {

@@ -3650,6 +3651,8 @@ 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;
+
  	/* Yak, is it really working this way? Some helper please? */
  	skb->len -= shiftlen;
  	skb->data_len -= shiftlen;

@@ -4017,6 +4020,8 @@ 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;

@@ -4251,7 +4256,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
  		skb_copy_from_linear_data_offset(head_skb, offset,
  						 skb_put(nskb, hsize), hsize);
 
-		skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
+		skb_shinfo(nskb)->flags |= (skb_shinfo(head_skb)->flags |
+					    skb_shinfo(frag_skb)->flags) &
  					   SKBFL_SHARED_FRAG;
 
  		if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))

@@ -4268,6 +4274,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
  				nfrags = skb_shinfo(list_skb)->nr_frags;
  				frag = skb_shinfo(list_skb)->frags;
  				frag_skb = list_skb;
+
+				skb_shinfo(nskb)->flags |= skb_shinfo(frag_skb)->flags & SKBFL_SHARED_FRAG;
+
  				if (!skb_headlen(list_skb)) {
  					BUG_ON(!nfrags);
  				} else {

@@ -4490,10 +4499,12 @@ 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;