Linux Kernel up to 6.18.15/6.19.5 io_zcrx_put_niov_uref double free

Summaryinfo

A vulnerability was found in Linux Kernel up to 6.18.15/6.19.5. It has been rated as critical. The affected element is the function io_zcrx_put_niov_uref. Performing a manipulation results in double free. This vulnerability is cataloged as CVE-2026-43121. There is no exploit available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability, which was classified as critical, was found in Linux Kernel up to 6.18.15/6.19.5. This affects the function io_zcrx_put_niov_uref. The manipulation with an unknown input leads to a double free vulnerability. CWE is classifying the issue as CWE-415. The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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]

The advisory is shared at git.kernel.org. This vulnerability is uniquely identified as CVE-2026-43121 since 05/01/2026. The exploitability is told to be difficult. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 05/18/2026).

The vulnerability scanner Nessus provides a plugin with the ID 313159 (Linux Distros Unpatched Vulnerability : CVE-2026-43121), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.18.16 or 6.19.6 eliminates this vulnerability. Applying the patch a94f096e28bfc7975163a6b80f1c8f323efe317a/485dc691257b96e6d3bdc25b0eff2daadcc5c46c/003049b1c4fb8aabb93febb7d1e49004f6ad653b is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (313159) and CERT Bund (WID-SEC-2026-1405). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Debian Linux
• Oracle Linux
• Microsoft Azure Linux
• Open Source Linux Kernel

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.18.0
• 6.18.1
• 6.18.2
• 6.18.3
• 6.18.4
• 6.18.5
• 6.18.6
• 6.18.7
• 6.18.8
• 6.18.9
• 6.18.10
• 6.18.11
• 6.18.12
• 6.18.13
• 6.18.14
• 6.18.15
• 6.19.0
• 6.19.1
• 6.19.2
• 6.19.3
• 6.19.4
• 6.19.5

License

• open-source

Website

• Vendor: https://www.kernel.org/

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Entryinfo

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