{"cve":"CVE-2026-43121","nvd":{"published":"2026-05-06T12:16:28.950","modified":"2026-06-17T10:48:58.537","status":"Analyzed","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nio_uring/zcrx: fix user_ref race between scrub and refill paths\n\nThe io_zcrx_put_niov_uref() function uses a non-atomic\ncheck-then-decrement pattern (atomic_read followed by separate\natomic_dec) to manipulate user_refs. This is serialized against other\ncallers by rq_lock, but io_zcrx_scrub() modifies the same counter with\natomic_xchg() WITHOUT holding rq_lock.\n\nOn SMP systems, the following race exists:\n\n  CPU0 (refill, holds rq_lock)          CPU1 (scrub, no rq_lock)\n  put_niov_uref:\n    atomic_read(uref) - 1\n    // window opens\n                                        atomic_xchg(uref, 0) - 1\n                                        return_niov_freelist(niov) [PUSH #1]\n    // window closes\n    atomic_dec(uref) - wraps to -1\n    returns true\n    return_niov(niov)\n    return_niov_freelist(niov)           [PUSH #2: DOUBLE-FREE]\n\nThe same niov is pushed to the freelist twice, causing free_count to\nexceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds\nwrite (a u32 value) past the kvmalloc'd freelist array into the adjacent\nslab object.\n\nFix this by replacing the non-atomic read-then-dec in\nio_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically\ntests and decrements user_refs. This makes the operation safe against\nconcurrent atomic_xchg from scrub without requiring scrub to acquire\nrq_lock.\n\n[pavel: removed a warning and a comment]","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H","score":4.7},"cwes":["CWE-362"],"references":["https://git.kernel.org/stable/c/003049b1c4fb8aabb93febb7d1e49004f6ad653b","https://git.kernel.org/stable/c/485dc691257b96e6d3bdc25b0eff2daadcc5c46c","https://git.kernel.org/stable/c/a94f096e28bfc7975163a6b80f1c8f323efe317a"],"products":["linux:linux_kernel"],"source":"nvd","synced_at":"2026-10-19T22:33:55.586217763+00:00"},"epss":{"cve":"CVE-2026-43121","score":0.00111,"percentile":0.01332,"date":"2026-11-01"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}