CVE-2026-43503

In the Linux kernel, the following vulnerability has been resolved:

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 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 skbs frag descriptors into the accumulators 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 ps frag_list. Downstream skb_segment() reads only skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skbs 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_skbs flag, and the inner switch that rebinds frag_skb to list_skb on head_skb-frags exhaustion does not fold the new frag_skbs flag into nskb. Fold frag_skbs flag at both sites so segments drawing frags from frag_list members carry the marker.

Affected Software

References

• https://git.kernel.org/stable/c/12401fcfb01f53ccc63ab0a3246570fe8f3105ee
• https://git.kernel.org/stable/c/179f1852bdedc300e373e807cc102cd81feff196
• https://git.kernel.org/stable/c/48f6a5356a33dd78e7144ae1faef95ffc990aae0
• https://git.kernel.org/stable/c/989214c66884d70716d83dc1d0bf5e16287bf349
• https://git.kernel.org/stable/c/9bc9d6d6967a2239aa57af2aa53554eddd640d20
• https://git.kernel.org/stable/c/fbeab9555564a1b98e8582cd106dfe46c4606991
• https://git.kernel.org/stable/c/fc6eb39c55e97df2f94ad974b8a5bbcd019da2c8
• https://git.kernel.org/stable/c/ff375cc75f9167168db38e0464a482d5fbc8d81d
• https://access.redhat.com/errata/RHSA-2026:19521
• https://access.redhat.com/errata/RHSA-2026:19540
• https://access.redhat.com/errata/RHSA-2026:19568
• https://access.redhat.com/errata/RHSA-2026:19569
• https://access.redhat.com/errata/RHSA-2026:19664
• https://access.redhat.com/errata/RHSA-2026:19666
• https://access.redhat.com/errata/RHSA-2026:19705
• https://access.redhat.com/errata/RHSA-2026:19711
• https://access.redhat.com/errata/RHSA-2026:19875
• https://access.redhat.com/errata/RHSA-2026:20051
• https://access.redhat.com/errata/RHSA-2026:20054
• https://access.redhat.com/errata/RHSA-2026:20087
• https://access.redhat.com/errata/RHSA-2026:20129
• https://access.redhat.com/errata/RHSA-2026:20130
• https://access.redhat.com/errata/RHSA-2026:20299
• https://access.redhat.com/errata/RHSA-2026:20593
• https://access.redhat.com/errata/RHSA-2026:21656
• https://access.redhat.com/errata/RHSA-2026:21690
• https://access.redhat.com/errata/RHSA-2026:21695
• https://access.redhat.com/errata/RHSA-2026:21702
• https://access.redhat.com/errata/RHSA-2026:23233
• https://access.redhat.com/errata/RHSA-2026:23240
• https://access.redhat.com/errata/RHSA-2026:23245
• https://access.redhat.com/errata/RHSA-2026:23468
• https://access.redhat.com/errata/RHSA-2026:23469
• https://access.redhat.com/errata/RHSA-2026:23470
• https://access.redhat.com/errata/RHSA-2026:23471
• https://access.redhat.com/errata/RHSA-2026:24814
• https://access.redhat.com/errata/RHSA-2026:25044
• https://access.redhat.com/errata/RHSA-2026:33486
• https://access.redhat.com/security/cve/CVE-2026-43503
• https://bugzilla.redhat.com/show_bug.cgi?id=2480902
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-43503.json