CVE-2025-21643

/// Technical description

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 thi

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References