# CVE-2026-43121: linux: In the Linux kernel, the following vulnerability has been resolved: io_uring/zcrx: fix user_ref race between scrub a... CVE-2026-43121 is fixed in 23 products on 1 platform, first by ALAS2023-2026-1702, released May 11, 2026. It is not reported as exploited. Patch Radar priority: Routine. - Web page: https://patchblox.com/security/cve/CVE-2026-43121 - Priority: Routine - CVSS: 4.7 (medium) - EPSS: 0.0011 - CISA KEV: not listed - Exploited (vendor report): no - Publicly disclosed: no - Published: May 6, 2026 ## Description In the Linux kernel, the following vulnerability has been resolved: io_uring/zcrx: fix user_ref race between scrub and refill paths The io_zcrx_put_niov_uref() function uses a non-atomic check-then-decrement pattern (atomic_read followed by separate atomic_dec) to manipulate user_refs. This is serialized against other callers by rq_lock, but io_zcrx_scrub() modifies the same counter with atomic_xchg() WITHOUT holding rq_lock. On SMP systems, the following race exists: CPU0 (refill, holds rq_lock) CPU1 (scrub, no rq_lock) put_niov_uref: atomic_read(uref) - 1 // window opens atomic_xchg(uref, 0) - 1 return_niov_freelist(niov) [PUSH #1] // window closes atomic_dec(uref) - wraps to -1 returns true return_niov(niov) return_niov_freelist(niov) [PUSH #2: DOUBLE-FREE] The same niov is pushed to the freelist twice, causing free_count to exceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds write (a u32 value) past the kvmalloc'd freelist array into the adjacent slab object. Fix this by replacing the non-atomic read-then-dec in io_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically tests and decrements user_refs. This makes the operation safe against concurrent atomic_xchg from scrub without requiring scrub to acquire rq_lock. [pavel: removed a warning and a comment] ## Where it's fixed | Platform | Product | Fixed in | Fix | Released | |---|---|---|---|---| | Linux | bpftool6.18 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | bpftool6.18-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel-livepatch-6.18.16-18.222 amzn 2023 | 1:1.0-0.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-debuginfo-common-aarch64 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-debuginfo-common-x86_64 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-devel amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-headers amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-libbpf amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-libbpf-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-libbpf-devel amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-libbpf-static amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-modules-extra amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-modules-extra-common amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-tools amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-tools-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | kernel6.18-tools-devel amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | linux Debian 14 | 6.19.6-1 | [DEBIAN-CVE-2026-43121](https://patchblox.com/security/data/fix/linux/debian-cve-2026-43121.md) | May 15, 2026 | | Linux | perf6.18 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | perf6.18-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | python3-perf6.18 amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | | Linux | python3-perf6.18-debuginfo amzn 2023 | 1:6.18.16-18.222.amzn2023 | [ALAS2023-2026-1702](https://patchblox.com/security/data/fix/linux/alas2023-2026-1702.md) | May 11, 2026 | --- Data as of 2026-11-08T19:12:23Z UTC. 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