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

btrfs: write-protect folios during data writeback

commit 095be159f3eb ("btrfs: unify folio dirty flag clearing") replaced
the folio_clear_dirty_for_io() call in extent_write_cache_pages() with a
plain folio_test_dirty() check. Besides clearing the dirty flag,
folio_clear_dirty_for_io() also calls folio_mkclean(), which write-protects
the shared mmap PTEs mapping the folio. Note that we still do call
folio_clear_dirty_for_io() later in submit_one_sector() when we clear
dirty on the last sector of the folio (the only sector for non-subpage
cases). But we lost this early call in extent_write_cache_pages().

Without the extra write-protection, a process with the file mmap-ed can
modify a sector while it is being used by writeback in a way that
expects a stable folio (checksumming, compressing, copying, etc...)
without faulting, which manifests as a handful of concrete bugs.

1. For large folios or subpage sectorsize, it is possible to submit a bio
which does not cover the whole folio. When this happens, we will have a
bio in flight for a folio that we have *not* called
folio_clear_dirty_for_io() on. If a task with an existing mmap-ed PTE
writes (without faulting..) in this window, it can result in
corruptions. If the write arrives while the checksumming or writing itself
is underway, this can result in an invalid checksum and later corruption
reports on read. If the write arrives after checksumming/writing is done
but before the last sector dirty is cleared, then the write is present
in page cache but doesn't affect the dirty tracking and will be lost
when the folio is fully finished being submitted and the dirty bit
is cleared. This results in losing the write even if fsync() is called.

2. For zoned submissions which are done in batch separate from the main
extent_writepage() loop, we also risk csum violations for those
submissions. Zoned writes are clamped to max_zone_append_size and are
not aligned with folios, so a submission can span two folios. The first
folio being processed in extent_write_cache_pages() will call
extent_write_locked_range() which will submit the partial range of the
next folio, while the rest of that folio could still be dirty. So
clearing dirty on the submitted sectors doesn't call
folio_clear_dirty_for_io() and we have the same issue. Since
extent_write_cache_pages() skips these batch submitted folios (they are
already marked for writeback from submission by the preceding folio), we
must add the extra write protection in lock_delalloc_folios().

3. For inline extents this will subtly risk losing writes that happen
after/while we copy the inline extent but before we clear dirty on
the folio.

4. For folios spanning EOF, mmap could tamper with the zeroed bytes past
EOF and cause them to be persisted where future faults would improperly
see them instead of zeros.

5. Finally, for compressed extents, we risk modifying the folios while we
work on compressing them which will result in corrupted compressed data.
Specifically, in run_delalloc_compressed() we queue up work to do
compress_file_range() in BTRFS_COMPRESSION_CHUNK_SIZE (512K) chunks which
will call btrfs_folio_clamp_clear_dirty() on the range. For non-subpage,
this will always clear the whole folio, safely. For subpage, we risk a
partial clear here as well. In particular, imagine a 2M folio broken up
into 512K chunks of work which might start compression work on one chunk
before all the chunks compress_file_range() workers have gotten far
enough to finish clearing all the dirty bitmaps of the folio and getting
to folio_clear_dirty_for_io(). Large folios on the edges of submission
ranges are similarly at risk to be only partly cleared.
This particular gap was introduced by a second patch in the same series:
commit a4ef54dbb576 ("btrfs: make extent_range_clear_dirty_for_io() to handle sector size < page size cases")

We cannot simply restore the call to folio_clear
---truncated---
Published: 2026-09-11
Score: 6.3 Medium
EPSS: < 1% Very Low
KEV: No
Impact: Data integrity and potential data loss
Action: Patch Immediately
AI Analysis

Impact

The vulnerability arises when the Linux kernel’s btrfs subsystem omits an early call to protect folios with an mmap‑ed file to modify sectors while checksumming, compressing, or writing them. This can corrupt checksums, lose writes, alter zeroed bytes past EOF, and produce corrupted compressed data, all of which lead to data integrity failures. The flaw maps to CWE‑413, uninitialized or incorrect data storage.

Affected Systems

The issue exists in all distributions that ship the kernel with btrfs support and have not yet applied the patch. No specific version range is provided, so any kernel containing this code may be affected until a fix is applied.

Risk and Exploitability

The CVSS base score of 6.3 indicates moderate severity. The EPSS score of < 1% reflects a very low probability of exploitation, and the vulnerability is not listed in the CISA KEV catalog, suggesting limited current exploitation activity. Exploitation requires a mapped file undergoing writeback, which typically occurs in privileged services or user data corruption and potential data loss; it does not grant code execution.

Generated by OpenCVE AI on September 15, 2026 at 19:47 UTC.

Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

  • Apply the latest Linux kernel update that contains the btrfs write‑protect fix for CVE‑2026‑89772.
  • Run a full btrfs scrub or checksum verification to detect and correct existing corruption.
  • If the kernel cannot be updated immediately, remount the affected Btrfs volumes in read‑only mode or restrict write access until the patch is applied to prevent further corruption.

Generated by OpenCVE AI on September 15, 2026 at 19:47 UTC.

Tracking

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Advisories

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History

Sat, 12 Sep 2026 00:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-413
References
Metrics threat_severity

None

 cvssV3_1

{'score': 6.3, 'vector': 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H'}

threat_severity

Moderate

Fri, 11 Sep 2026 23:45:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: btrfs: write-protect folios during data writeback commit 095be159f3eb ("btrfs: unify folio dirty flag clearing") replaced the folio_clear_dirty_for_io() call in extent_write_cache_pages() with a plain folio_test_dirty() check. Besides clearing the dirty flag, folio_clear_dirty_for_io() also calls folio_mkclean(), which write-protects the shared mmap PTEs mapping the folio. Note that we still do call folio_clear_dirty_for_io() later in submit_one_sector() when we clear dirty on the last sector of the folio (the only sector for non-subpage cases). But we lost this early call in extent_write_cache_pages(). Without the extra write-protection, a process with the file mmap-ed can modify a sector while it is being used by writeback in a way that expects a stable folio (checksumming, compressing, copying, etc...) without faulting, which manifests as a handful of concrete bugs. 1. For large folios or subpage sectorsize, it is possible to submit a bio which does not cover the whole folio. When this happens, we will have a bio in flight for a folio that we have *not* called folio_clear_dirty_for_io() on. If a task with an existing mmap-ed PTE writes (without faulting..) in this window, it can result in corruptions. If the write arrives while the checksumming or writing itself is underway, this can result in an invalid checksum and later corruption reports on read. If the write arrives after checksumming/writing is done but before the last sector dirty is cleared, then the write is present in page cache but doesn't affect the dirty tracking and will be lost when the folio is fully finished being submitted and the dirty bit is cleared. This results in losing the write even if fsync() is called. 2. For zoned submissions which are done in batch separate from the main extent_writepage() loop, we also risk csum violations for those submissions. Zoned writes are clamped to max_zone_append_size and are not aligned with folios, so a submission can span two folios. The first folio being processed in extent_write_cache_pages() will call extent_write_locked_range() which will submit the partial range of the next folio, while the rest of that folio could still be dirty. So clearing dirty on the submitted sectors doesn't call folio_clear_dirty_for_io() and we have the same issue. Since extent_write_cache_pages() skips these batch submitted folios (they are already marked for writeback from submission by the preceding folio), we must add the extra write protection in lock_delalloc_folios(). 3. For inline extents this will subtly risk losing writes that happen after/while we copy the inline extent but before we clear dirty on the folio. 4. For folios spanning EOF, mmap could tamper with the zeroed bytes past EOF and cause them to be persisted where future faults would improperly see them instead of zeros. 5. Finally, for compressed extents, we risk modifying the folios while we work on compressing them which will result in corrupted compressed data. Specifically, in run_delalloc_compressed() we queue up work to do compress_file_range() in BTRFS_COMPRESSION_CHUNK_SIZE (512K) chunks which will call btrfs_folio_clamp_clear_dirty() on the range. For non-subpage, this will always clear the whole folio, safely. For subpage, we risk a partial clear here as well. In particular, imagine a 2M folio broken up into 512K chunks of work which might start compression work on one chunk before all the chunks compress_file_range() workers have gotten far enough to finish clearing all the dirty bitmaps of the folio and getting to folio_clear_dirty_for_io(). Large folios on the edges of submission ranges are similarly at risk to be only partly cleared. This particular gap was introduced by a second patch in the same series: commit a4ef54dbb576 ("btrfs: make extent_range_clear_dirty_for_io() to handle sector size < page size cases") We cannot simply restore the call to folio_clear ---truncated---
Title btrfs: write-protect folios during data writeback
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-09-11T19:47:11.125Z

Reserved: 2026-09-11T19:38:34.765Z

Link: CVE-2026-89772

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-09-11T20:20:08.577

Modified: 2026-09-11T20:20:08.577

Link: CVE-2026-89772

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-09-11T19:47:11Z

Links: CVE-2026-89772 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-09-15T20:00:08Z

Weaknesses
  • CWE-413

    Improper Resource Locking