net: skbuff: propagate shared-frag marker through frag-transfer helpers
Replace the partial fix shipped in 6.12.0-227 (which was a backport of Hyunwoo Kim's v1 patch covering only __pskb_copy_fclone() and skb_try_coalesce()) with a backport of the upstream v3 patch: https://lore.kernel.org/all/agW4vC0r8QOUKtRT@v4bel/ v3 also covers skb_shift() (the new hunk added in v2) and skb_gro_receive() / skb_gro_receive_list() (the audit follow-up suggested by Sultan Alsawaf in agVpIsaSherjHTYg@sultan-box). All five sites moved frag descriptors between skbs without propagating the SKBFL_SHARED_FRAG marker, so destinations could end up referencing externally-owned or page-cache-backed pages while reporting skb_has_shared_frag() as false. Combined with ESP-over-UDP and UDP GRO, or any nf_dup_ipv4 / xt_TEE caller, this lets an unprivileged user trigger in-place ESP decryption over root-owned page-cache pages (CVE-2026-46300, "Fragnesia").
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@ -1,27 +1,27 @@
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From: Andrew Lukoshko <alukoshko@almalinux.org>
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Subject: [PATCH AlmaLinux 10] net: skbuff: propagate shared-frag marker through pskb_copy()
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From: Eduard Abdullin <eabdullin@almalinux.org>
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Subject: [PATCH AlmaLinux 10] net: skbuff: propagate shared-frag marker through frag-transfer helpers
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Backport of upstream patch posted at
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https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/
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(sibling to the xfrm/esp shared-frag fix upstream commit f4c50a4034e6,
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already merged into 6.12.0-124.56.1 via the c10s import).
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Backport of upstream v3 patch posted at
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https://lore.kernel.org/all/agW4vC0r8QOUKtRT@v4bel/
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(sibling to upstream commit f4c50a4034e6 "xfrm: esp: avoid in-place
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decrypt on shared skb frags").
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__pskb_copy_fclone() shallow-copies the source's frag descriptors and
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bumps each page's refcount via skb_frag_ref(), then defers the rest
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of the shinfo metadata to skb_copy_header(). That helper only carries
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over gso_{size,segs,type} and never touches skb_shinfo()->flags, so
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the destination skb keeps a reference to the same externally-owned or
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page-cache-backed pages while reporting skb_has_shared_frag() as
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false.
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Three frag-transfer helpers (__pskb_copy_fclone(), skb_try_coalesce(),
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and skb_shift()) and the GRO accumulator helpers (skb_gro_receive()
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and skb_gro_receive_list()) fail to propagate the SKBFL_SHARED_FRAG
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bit in skb_shinfo()->flags when moving frag descriptors from source
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to destination. As a result, the destination skb keeps a reference
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to the same externally-owned or page-cache-backed pages while
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reporting skb_has_shared_frag() as false.
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The mismatch is harmful in any in-place writer that uses
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skb_has_shared_frag() to decide whether shared pages must be detoured
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through skb_cow_data(). ESP input is one such writer (esp4.c,
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esp6.c), and a single nft 'dup to <local>' rule -- or any other
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nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d
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skb in esp_input() with the marker stripped, letting an unprivileged
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user write into the page cache of a root-owned read-only file via
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authencesn-ESN stray writes.
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esp6.c). Combined with an nft 'dup to <local>' rule, an
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nf_dup_ipv4() / xt_TEE caller, or ESP-over-UDP with UDP GRO, this
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lets an unprivileged user write into the page cache of a root-owned
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read-only file via authencesn-ESN stray writes (CVE-2026-46300,
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"Fragnesia").
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Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors
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were actually moved from the source. skb_copy() and skb_copy_expand()
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@ -29,18 +29,50 @@ share skb_copy_header() too but linearize all paged data into freshly
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allocated head storage and emerge with nr_frags == 0, so
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skb_has_shared_frag() returns false on its own; they need no change.
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Verified to apply with `patch -p1 -F0` (no offset, no fuzz, no rejects)
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against kernel-6.12.0-124.56.1.el10_1.
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The same omission exists in skb_gro_receive() and
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skb_gro_receive_list(). The former moves the incoming skb's frag
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descriptors into the accumulator's last sub-skb via two paths plus a
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"merge:" fallback that chains the whole skb onto frag_list; the
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latter chains the incoming skb whole onto p's frag_list. Downstream
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skb_segment() reads only skb_shinfo(p)->flags, and skb_segment_list()
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reuses each sub-skb's shinfo as the nskb -- both p and lp must carry
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the marker.
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Fixes: cef401de7be8 ("net: fix possible wrong checksum generation")
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Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
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Reported-by: Sultan Alsawaf <sultan@kerneltoast.com>
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Reported-by: William Bowling <vakzz@zellic.io>
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Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
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Signed-off-by: Andrew Lukoshko <alukoshko@almalinux.org>
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Signed-off-by: Eduard Abdullin <eabdullin@almalinux.org>
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---
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net/core/skbuff.c | 3 +++
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1 file changed, 3 insertions(+)
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net/core/gro.c | 4 ++++
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net/core/skbuff.c | 5 +++++
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2 files changed, 9 insertions(+)
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--- a/net/core/gro.c
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+++ b/net/core/gro.c
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@@ -213,10 +213,12 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
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p->data_len += len;
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p->truesize += delta_truesize;
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p->len += len;
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+ skb_shinfo(p)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
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if (lp != p) {
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lp->data_len += len;
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lp->truesize += delta_truesize;
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lp->len += len;
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+ skb_shinfo(lp)->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
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}
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NAPI_GRO_CB(skb)->same_flow = 1;
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return 0;
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@@ -244,6 +246,8 @@ int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
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p->truesize += skb->truesize;
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p->len += skb->len;
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+ skb_shinfo(p)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
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+
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NAPI_GRO_CB(skb)->same_flow = 1;
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return 0;
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--- a/net/core/skbuff.c
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+++ b/net/core/skbuff.c
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@@ -2123,6 +2123,7 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom,
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@ -51,7 +83,16 @@ Signed-off-by: Andrew Lukoshko <alukoshko@almalinux.org>
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}
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if (skb_has_frag_list(skb)) {
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@@ -6028,6 +6029,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
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@@ -4198,6 +4199,8 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen)
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tgt->ip_summed = CHECKSUM_PARTIAL;
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skb->ip_summed = CHECKSUM_PARTIAL;
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+ skb_shinfo(tgt)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
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+
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skb_len_add(skb, -shiftlen);
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skb_len_add(tgt, shiftlen);
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@@ -6028,6 +6031,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
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from_shinfo->frags,
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from_shinfo->nr_frags * sizeof(skb_frag_t));
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to_shinfo->nr_frags += from_shinfo->nr_frags;
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@ -4558,6 +4558,9 @@ fi\
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#
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#
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%changelog
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* Thu May 14 2026 Eduard Abdullin <eabdullin@almalinux.org> - 6.12.0-227
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- net: skbuff: propagate shared-frag marker through frag-transfer helpers
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* Wed May 13 2026 Andrew Lukoshko <alukoshko@almalinux.org> - 6.12.0-227
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- xfrm: esp: avoid in-place decrypt on shared skb frags (CVE-2026-43284)
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- rxrpc: linearize incoming DATA packet when it has paged frags (CVE-2026-43500)
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