🛡️ CVE-2025-21643 — kernel

🟡 CVSS 5.5 — Medium ✅ No Known Exploit NVD
5.5
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

netfs: Fix kernel async DIO

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix kernel async DIO

Netfslib needs to be able to handle kernel-initiated asynchronous DIO that

is supplied with a bio_vec[] array. Currently, because of the async flag,

this gets passed to netfs_extract_user_iter() which throws a warning and

fails because it only handles IOVEC and UBUF iterators. This can be

triggered through a combination of cifs and a loopback blockdev with

something like:

mount //my/cifs/share /foo

dd if=/dev/zero of=/foo/m0 bs=4K count=1K

losetup --sector-size 4096 --direct-io=on /dev/loop2046 /foo/m0

echo hello >/dev/loop2046

This causes the following to appear in syslog:

WARNING: CPU: 2 PID: 109 at fs/netfs/iterator.c:50 netfs_extract_user_iter+0x170/0x250 [netfs]

and the write to fail.

Fix this by removing the check in netfs_unbuffered_write_iter_locked() that

causes async kernel DIO writes to be handled as userspace writes. Note

that this change relies on the kernel caller maintaining the existence of

the bio_vec array (or kvec[] or folio_queue) until the op is complete.

How this vulnerability can be exploited

This issue can be reached with local access to the system, attack complexity is low, an attacker needs low-level privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity none, availability high.

Affected software

CVE-2025-21643 is recorded against 2 packages.

  • kernel (from 6.8.0 up to 6.12.10)
  • linux-kernel

Timeline and source

Published on 19 January 2025 and last revised on 15 July 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

git.kernel.org (Web)
git.kernel.org (Web)
github.com (Advisory)
nvd.nist.gov (Advisory)
git.kernel.org (Package)

CVE-2025-21643 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity Medium
CVSS Score 5.5
CVSS Vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
CWE N/A
Public Exploit ✅ No
Source NVD
Published 2025-01-19
Updated 2026-08-12
Modified 2026-07-15

Affected Packages

Software From version Fixed in
kernel 6.8.0 6.12.10
linux-kernel — —

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