GHSA-494P-Q444-9XF7
Vulnerability from github – Published: 2026-05-26 13:30 – Updated: 2026-05-30 12:30In 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 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.
{
"affected": [],
"aliases": [
"CVE-2026-43503"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-05-23T12:17:02Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: skbuff: propagate shared-frag marker through frag-transfer helpers\n\nTwo frag-transfer helpers (__pskb_copy_fclone() and skb_shift()) fail\nto propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()-\u003eflags when\nmoving frags from source to destination. __pskb_copy_fclone() defers\nthe rest of the shinfo metadata to skb_copy_header() after copying\nfrag descriptors, but that helper only carries over gso_{size,segs,\ntype} and never touches skb_shinfo()-\u003eflags; skb_shift() moves frag\ndescriptors directly and leaves flags untouched. As a result, the\ndestination skb keeps a reference to the same externally-owned or\npage-cache-backed pages while reporting skb_has_shared_frag() as\nfalse.\n\nThe mismatch is harmful in any in-place writer that uses\nskb_has_shared_frag() to decide whether shared pages must be detoured\nthrough skb_cow_data(). ESP input is one such writer (esp4.c,\nesp6.c), and a single nft \u0027dup to \u003clocal\u003e\u0027 rule -- or any other\nnf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()\u0027d\nskb in esp_input() with the marker stripped, letting an unprivileged\nuser write into the page cache of a root-owned read-only file via\nauthencesn-ESN stray writes.\n\nSet SKBFL_SHARED_FRAG on the destination whenever frag descriptors\nwere actually moved from the source. skb_copy() and skb_copy_expand()\nshare skb_copy_header() too but linearize all paged data into freshly\nallocated head storage and emerge with nr_frags == 0, so\nskb_has_shared_frag() returns false on its own; they need no change.\n\nThe same omission exists in skb_gro_receive() and skb_gro_receive_list().\nThe former moves the incoming skb\u0027s frag descriptors into the\naccumulator\u0027s last sub-skb via two paths (a direct frag-move loop and\nthe head_frag + memcpy path); the latter chains the incoming skb whole\nonto p\u0027s frag_list. Downstream skb_segment() reads only\nskb_shinfo(p)-\u003eflags, and skb_segment_list() reuses each sub-skb\u0027s\nshinfo as the nskb -- both p and lp must carry the marker.\n\nThe same omission also exists in tcp_clone_payload(), which builds an\nMTU probe skb by moving frag descriptors from skbs on sk_write_queue\ninto a freshly allocated nskb. The helper falls into the same family\nand warrants the same fix for consistency; no TCP TX-side in-place\nwriter is currently known to reach a user page through this gap, but\na future consumer depending on the marker would regress silently.\n\nThe same omission exists in skb_segment(): the per-iteration flag\nmerge takes only head_skb\u0027s flag, and the inner switch that rebinds\nfrag_skb to list_skb on head_skb-frags exhaustion does not fold the\nnew frag_skb\u0027s flag into nskb. Fold frag_skb\u0027s flag at both sites\nso segments drawing frags from frag_list members carry the marker.",
"id": "GHSA-494p-q444-9xf7",
"modified": "2026-05-30T12:30:23Z",
"published": "2026-05-26T13:30:24Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-43503"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/12401fcfb01f53ccc63ab0a3246570fe8f3105ee"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/179f1852bdedc300e373e807cc102cd81feff196"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/48f6a5356a33dd78e7144ae1faef95ffc990aae0"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/989214c66884d70716d83dc1d0bf5e16287bf349"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9bc9d6d6967a2239aa57af2aa53554eddd640d20"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/fbeab9555564a1b98e8582cd106dfe46c4606991"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/fc6eb39c55e97df2f94ad974b8a5bbcd019da2c8"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ff375cc75f9167168db38e0464a482d5fbc8d81d"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.