LiveActive security incident?Get immediate response
CVE Record

CVE-2025-21643: 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.

UnknownCVSS not scoredNot KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

This Linux kernel issue can make certain network-filesystem direct writes fail. The documented trigger uses CIFS storage with a loopback block device and direct I/O. The public record describes warning logs and failed writes, not remote compromise or data theft.

Executive priority

Treat as routine kernel maintenance unless CIFS-backed loop devices or direct I/O are used in production. For those systems, schedule faster remediation because failed writes can affect service reliability.

Technical view

Netfs async kernel direct I/O supplied with bio_vec arrays was incorrectly routed through user-iterator handling. netfs_extract_user_iter() only handled IOVEC and UBUF iterators, causing a warning and failed write. Stable commits correct netfs_unbuffered_write_iter_locked() handling.

Likely exposure

Exposure appears limited to affected Linux kernels using netfs paths, especially CIFS-backed files combined with loopback block devices and direct I/O. General Linux hosts not using this storage pattern may have little practical exposure based on the provided sources.

Exploitation context

The source bundle provides a local reproduction scenario and states KEV is false. It does not cite active exploitation, public weaponization, privilege escalation, remote attack, or confidentiality impact.

Researcher notes

Evidence supports an availability or reliability issue in a specific netfs async DIO path. Severity, CVSS, CWE, and broader impact are not provided. Avoid assuming exploitability beyond the documented warning and write failure.

Mitigation direction

  • Update to a kernel containing the referenced stable fixes.
  • Check your Linux distribution advisory for the backported fixed package.
  • Prioritize systems using CIFS, loopback block devices, or direct I/O storage paths.
  • Monitor for netfs_extract_user_iter warnings and failed writes until patched.

Validation and detection

  • Inventory Linux kernel versions against the affected range in the CVE record.
  • Confirm whether vendor kernels include the referenced stable commits.
  • Identify CIFS mounts and loop devices configured with direct I/O.
  • Review syslog or kernel logs for the documented netfs warning.
  • Retest affected storage workflows after updating the kernel.
Prepared
Confidence
medium
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cve · low confidence lookup

CVE-2025-21643 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
0ADP providers
3Source links

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux153a9961b551101cd38e94e26cd92fbfd198b19b, 153a9961b551101cd38e94e26cd92fbfd198b19bunaffected
LinuxLinux6.8, 0, 6.12.10, 6.13affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.