GHSA-F26P-P2HG-C5QG
Vulnerability from github – Published: 2026-05-06 12:30 – Updated: 2026-05-11 15:32In the Linux kernel, the following vulnerability has been resolved:
kcm: fix zero-frag skb in frag_list on partial sendmsg error
Syzkaller reported a warning in kcm_write_msgs() when processing a message with a zero-fragment skb in the frag_list.
When kcm_sendmsg() fills MAX_SKB_FRAGS fragments in the current skb, it allocates a new skb (tskb) and links it into the frag_list before copying data. If the copy subsequently fails (e.g. -EFAULT from user memory), tskb remains in the frag_list with zero fragments:
head skb (msg being assembled, NOT yet in sk_write_queue) +-----------+ | frags[17] | (MAX_SKB_FRAGS, all filled with data) | frag_list-+--> tskb +-----------+ +----------+ | frags[0] | (empty! copy failed before filling) +----------+
For SOCK_SEQPACKET with partial data already copied, the error path saves this message via partial_message for later completion. For SOCK_SEQPACKET, sock_write_iter() automatically sets MSG_EOR, so a subsequent zero-length write(fd, NULL, 0) completes the message and queues it to sk_write_queue. kcm_write_msgs() then walks the frag_list and hits:
WARN_ON(!skb_shinfo(skb)->nr_frags)
TCP has a similar pattern where skbs are enqueued before data copy and cleaned up on failure via tcp_remove_empty_skb(). KCM was missing the equivalent cleanup.
Fix this by tracking the predecessor skb (frag_prev) when allocating a new frag_list entry. On error, if the tail skb has zero frags, use frag_prev to unlink and free it in O(1) without walking the singly-linked frag_list. frag_prev is safe to dereference because the entire message chain is only held locally (or in kcm->seq_skb) and is not added to sk_write_queue until MSG_EOR, so the send path cannot free it underneath us.
Also change the WARN_ON to WARN_ON_ONCE to avoid flooding the log if the condition is somehow hit repeatedly.
There are currently no KCM selftests in the kernel tree; a simple reproducer is available at [1].
[1] https://gist.github.com/mrpre/a94d431c757e8d6f168f4dd1a3749daa
{
"affected": [],
"aliases": [
"CVE-2026-43244"
],
"database_specific": {
"cwe_ids": [
"CWE-401"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-05-06T12:16:44Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nkcm: fix zero-frag skb in frag_list on partial sendmsg error\n\nSyzkaller reported a warning in kcm_write_msgs() when processing a\nmessage with a zero-fragment skb in the frag_list.\n\nWhen kcm_sendmsg() fills MAX_SKB_FRAGS fragments in the current skb,\nit allocates a new skb (tskb) and links it into the frag_list before\ncopying data. If the copy subsequently fails (e.g. -EFAULT from\nuser memory), tskb remains in the frag_list with zero fragments:\n\n head skb (msg being assembled, NOT yet in sk_write_queue)\n +-----------+\n | frags[17] | (MAX_SKB_FRAGS, all filled with data)\n | frag_list-+--\u003e tskb\n +-----------+ +----------+\n | frags[0] | (empty! copy failed before filling)\n +----------+\n\nFor SOCK_SEQPACKET with partial data already copied, the error path\nsaves this message via partial_message for later completion. For\nSOCK_SEQPACKET, sock_write_iter() automatically sets MSG_EOR, so a\nsubsequent zero-length write(fd, NULL, 0) completes the message and\nqueues it to sk_write_queue. kcm_write_msgs() then walks the\nfrag_list and hits:\n\n WARN_ON(!skb_shinfo(skb)-\u003enr_frags)\n\nTCP has a similar pattern where skbs are enqueued before data copy\nand cleaned up on failure via tcp_remove_empty_skb(). KCM was\nmissing the equivalent cleanup.\n\nFix this by tracking the predecessor skb (frag_prev) when allocating\na new frag_list entry. On error, if the tail skb has zero frags,\nuse frag_prev to unlink and free it in O(1) without walking the\nsingly-linked frag_list. frag_prev is safe to dereference because\nthe entire message chain is only held locally (or in kcm-\u003eseq_skb)\nand is not added to sk_write_queue until MSG_EOR, so the send path\ncannot free it underneath us.\n\nAlso change the WARN_ON to WARN_ON_ONCE to avoid flooding the log\nif the condition is somehow hit repeatedly.\n\nThere are currently no KCM selftests in the kernel tree; a simple\nreproducer is available at [1].\n\n[1] https://gist.github.com/mrpre/a94d431c757e8d6f168f4dd1a3749daa",
"id": "GHSA-f26p-p2hg-c5qg",
"modified": "2026-05-11T15:32:08Z",
"published": "2026-05-06T12:30:34Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-43244"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/7af58f76e4b404a74c836881a845e6652db8a09f"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9ea3671d70ee07480d80bebe86696397c4e99fb7"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b1e3edf688a88c1a3ac41657055d9c136a08cd25"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ca220141fa8ebae09765a242076b2b77338106b0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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