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

netfs: Fix write streaming disablement if fd open O_RDWR

In netfs_perform_write(), "write streaming" (the caching of dirty data in
dirty but !uptodate folios) is performed to avoid the need to read data
that is just going to get immediately overwritten. However, this is/will
be disabled in three circumstances: if the fd is open O_RDWR, if fscache is
in use (as we need to round out the blocks for DIO) or if content
encryption is enabled (again for rounding out purposes).

The idea behind disabling it if the fd is open O_RDWR is that we'd need to
flush the write-streaming page before we could read the data, particularly
through mmap. But netfs now fills in the gaps if ->read_folio() is called
on the page, so that is unnecessary. Further, this doesn't actually work
if a separate fd is open for reading.

Fix this by removing the check for O_RDWR, thereby allowing streaming
writes even when we might read.

This caused a number of problems with the generic/522 xfstest, but those
are now fixed.
Published: 2026-07-19
Score: 7.3 High
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

Impact

In the Linux kernel, the netfs subsystem previously disabled the write‑streaming optimization when a file descriptor was opened with O_RDWR, as well as under fscache and encryption. The write‑streaming mechanism caches dirty pages that are not yet up‑to‑date, improving performance. The disabling logic was found to be incorrect because the kernel now fills gaps on read. Consequently, this oversight caused failures in the generic/522 xfstest suite. The issue is classified as CWE‑253 and receives a CVSS score of 7.3.

Affected Systems

The affected system is the Linux operating system kernel. Any kernel version prior to the commit that removes the O_RDWR check for write streaming may be impacted.

Risk and Exploitability

The vulnerability is a kernel logic error that could be exercised by any process that performs file operations via the netfs subsystem. No exploit has been publicly disclosed. With an EPSS score of less than 1 % and no listing in CISA's KEV catalog, the likelihood of real‑world exploitation is low, but the CVSS score reflects a high severity potential for performance degradation and I/O inconsistencies.

Generated by OpenCVE AI on July 30, 2026 at 20:08 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Upgrade the Linux kernel to a release that incorporates the upstream commit removing the O_RDWR check for write streaming in netfs_perform_write()
  • If an immediate kernel upgrade is not possible, apply the patch from the referenced commit and rebuild the kernel before rebooting
  • After updating, run regressions or suitable I/O tests to confirm that write‑streaming operates correctly and that no test failures similar to the generic/522 xfstest occur

Generated by OpenCVE AI on July 30, 2026 at 20:08 UTC.

Tracking

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Advisories
Source ID Title
Ubuntu USN Ubuntu USN USN-8593-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-8603-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-8618-1 Linux kernel vulnerabilities
History

Mon, 20 Jul 2026 14:45:00 +0000

Type Values Removed Values Added
Metrics cvssV3_1

{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}

 cvssV3_1

{'score': 7.3, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:L'}

Mon, 20 Jul 2026 12:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-253
References
Metrics threat_severity

None

 cvssV3_1

{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}

threat_severity

Low

Sun, 19 Jul 2026 16:15:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: netfs: Fix write streaming disablement if fd open O_RDWR In netfs_perform_write(), "write streaming" (the caching of dirty data in dirty but !uptodate folios) is performed to avoid the need to read data that is just going to get immediately overwritten. However, this is/will be disabled in three circumstances: if the fd is open O_RDWR, if fscache is in use (as we need to round out the blocks for DIO) or if content encryption is enabled (again for rounding out purposes). The idea behind disabling it if the fd is open O_RDWR is that we'd need to flush the write-streaming page before we could read the data, particularly through mmap. But netfs now fills in the gaps if ->read_folio() is called on the page, so that is unnecessary. Further, this doesn't actually work if a separate fd is open for reading. Fix this by removing the check for O_RDWR, thereby allowing streaming writes even when we might read. This caused a number of problems with the generic/522 xfstest, but those are now fixed.
Title netfs: Fix write streaming disablement if fd open O_RDWR
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: 2026-08-05T12:39:49.120Z

Reserved: 2026-07-19T07:54:57.038Z

Link: CVE-2026-64158

cve-icon Vulnrichment

No data.

cve-icon NVD

No data.

cve-icon Redhat

Severity : Low

Publid Date: 2026-07-19T00:00:00Z

Links: CVE-2026-64158 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-07-30T20:15:05Z

Weaknesses
  • CWE-253

    Incorrect Check of Function Return Value