# CVE-2026-92501: ext4: drain in-flight DIO before buffered write fallback

> A race condition in the Linux kernel ext4 filesystem can cause I/O errors and potential denial of service during concurrent direct and buffered writes.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102053, BELL-CVE-2026-92501, DEBIAN-CVE-2026-92501, GHSA-h82g-vj5r-m4jv, UBUNTU-CVE-2026-92501

## What is CVE-2026-92501?

CVE-2026-92501 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

ext4: drain in-flight DIO before buffered write fallback

generic/746 started failing intermittently on ext3 (no-extent inodes).
The test triggers 'Page cache invalidation failure on direct I/O'
warnings and subsequent fsync returns -EIO. Adding a 50ms delay
between ext4_buffered_write_iter() and filemap_write_and_wait_range()
in ext4_dio_write_iter() makes the race almost always reproducible.

On no-extent inodes, DIO writes to holes cannot use unwritten extents,
so ext4_iomap_alloc() leaves m_flags=0 and ext4_map_blocks() returns 0.
The iomap layer then returns -ENOTBLK, causing fallback to buffered I/O.

The fallback path in ext4_dio_write_iter() calls
ext4_buffered_write_iter() which dirties pages, then does flush and
invalidate. However, there's an unprotected window between
ext4_buffered_write_iter() returning (with inode lock released) and
the subsequent flush+invalidate.

Concurrent async DIO completions from other threads can run
kiocb_invalidate_post_direct_write() during this window. If pages have
been re-dirtied, post-invalidation finds dirty pages and triggers the
warning, setting -EIO in the error sequence.

Consider a file with two 4k extents: [hole][written]. Thread A does
DIO to the written extent, while thread B does DIO spanning both:

  kworker A (4k DIO, allocated block)    kworker B (8k DIO, fallback)
  -----------------------------------    ----------------------------
  inode_lock_shared()                    inode_lock_shared()
  iomap_dio_rw():                        iomap_dio_rw():
    kiocb_invalidate_pages -> clean        iomap_begin -> -ENOTBLK
    submit_bio (async)                     dio->size = 0
  inode_unlock_shared()                  inode_unlock_shared()

  [bio pending in block layer]           /* fallback: lock released */
                                         ext4_buffered_write_iter()
                                           inode_lock(exclusive)
                                           generic_perform_write()
                                             -> dirty pages [0, 8k]
                                           inode_unlock(exclusive)

                                         /* pages dirty, no lock */
  [bio completes]                        filemap_write_and_wait_range()
  iomap_dio_complete()                     -> flush dirty pages
    kiocb_invalidate_post_direct_write() invalidate_mapping_pages()
      invalidate_inode_pages2_range()
      -> finds dirty page!
      -> dio_warn_stale_pagecache()
      -> errseq_set(-EIO)

This issue can be triggered through normal I/O paths, not just
intentionally overlapping DIO writes from userspace. For example,
generic/746 uses a loop device where multiple kworkers issue concurrent
I/O to the backing file. Additionally, when block_size < folio_size,
non-overlapping DIO writes that share a large folio can also trigger
the race.

Add inode_dio_wait() in ext4_buffered_write_iter() before
ext4_write_checks() to drain all in-flight DIO. This ensures that
all DIO clears existing pages before submitting IO (via
kiocb_invalidate_pages()), all BIO waits for all DIO to complete
(via inode_dio_wait()), and ext4_write_checks() observes the inode
size after all completed DIO so that ext4_block_zero_eof() does not
race with in-flight DIO, thus eliminating the race.

## Which versions are affected by CVE-2026-92501?


## How do I fix CVE-2026-92501?

Apply the vendor's patch.

## Is CVE-2026-92501 being exploited?

There is no known exploitation of CVE-2026-92501 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-92501 and fix it if it is affected.

What it is: CVE-2026-92501 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-92501 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-92501/ (Markdown: https://insights.sbomify.com/CVE-2026-92501/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-92501
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-92501
- OSV: https://osv.dev/vulnerability/CVE-2026-92501

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-92501/
Summaries are generated and may contain mistakes; the official description above is the record.