CVE-2026-43121

Published May 6, 2026

Last updated a month ago

Overview

AI description

Automated description summarized from trusted sources.

CVE-2026-43121 is a vulnerability identified in the Linux kernel, specifically within the `io_uring/zcrx` component. The flaw stems from a race condition in the `io_zcrx_put_niov_uref()` function, which utilizes a non-atomic check-then-decrement pattern to manage `user_refs`. This non-atomic operation, when combined with `io_zcrx_scrub()` modifying the same counter without holding the necessary lock, creates a window for a race condition. This race condition can lead to a double-free situation, where the same `niov` (a structure related to I/O vectors) is pushed to the freelist twice. Subsequently, this can result in an out-of-bounds write past the `kvmalloc`'d freelist array into an adjacent slab object. The issue is resolved by replacing the non-atomic read-then-decrement with an atomic compare-and-swap loop, ensuring safe concurrent operations.

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]
Source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
NVD status: Analyzed
Products: linux_kernel

Risk scores

CVSS 3.1

Type: Primary
Base score: 4.7
Impact score: 3.6
Exploitability score: 1
Vector string: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Severity: MEDIUM

Weaknesses

nvd@nist.gov: CWE-362

Hype score: Not currently trending

Configurations

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References

Sources include official advisories and independent security research.