{"id":"CVE-2026-43244","summary":"kcm: fix zero-frag skb in frag_list on partial sendmsg error","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","modified":"2026-06-18T03:57:22.103184195Z","published":"2026-05-06T11:28:36.946Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43244.json"},"references":[{"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"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43244.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-43244"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"ab7ac4eb9832e32a09f4e8042705484d2fb0aad3"},{"fixed":"9ea3671d70ee07480d80bebe86696397c4e99fb7"},{"fixed":"b1e3edf688a88c1a3ac41657055d9c136a08cd25"},{"fixed":"7af58f76e4b404a74c836881a845e6652db8a09f"},{"fixed":"ca220141fa8ebae09765a242076b2b77338106b0"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-43244.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.6.0"},{"fixed":"6.12.75"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.16"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"6.19.6"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-43244.json"}}],"schema_version":"1.7.5"}