FROMLIST: net: skbuff: propagate shared-frag marker through frag-transfer helpers

maillist inclusion
category: bugfix
CVE: CVE-2026-46300

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: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Suggested-by: Sultan Alsawaf <sultan@kerneltoast.com>
Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://github.com/v12-security/pocs/tree/main/fragnesia
Link: https://x.com/v12sec/status/2054491454064746629
Link: https://lists.openwall.net/netdev/2026/05/13/79
Link: https://lore.kernel.org/all/20260513041635.1289541-1-vakzz@zellic.io/
Link: https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: V4bel

net/core/gro.c

@@ -215,10 +215,12 @@ 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;
@@ -246,6 +248,8 @@ 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

@@ -2188,6 +2188,7 @@ 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)) {
@@ -4301,6 +4302,8 @@ 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);
 

net/ipv4/tcp_output.c

@@ -2541,6 +2541,7 @@ 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) +