CVE-2026-72132: NFS: Charge unstable writes by request size, not folio size

CVE-2026-72132: NFS: Charge unstable writes by request size, not folio size

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

NFS: Charge unstable writes by request size, not folio size

nfs_folio_mark_unstable() and nfs_folio_clear_commit() charge and
uncharge NR_WRITEBACK/WB_WRITEBACK by folio_nr_pages(folio) once per
*request* added to or removed from a commit list. This is correct only
when a folio has a single associated request. When pg_test splits a
folio into N sub-folio requests (e.g. pNFS flexfiles striping with a
stripe unit smaller than the folio size, or plain wsize-limited
splitting), each of the N requests independently charges the whole
folio's page count, inflating the accounting by a factor of N per
folio. With large folios and small stripe units this reaches multiple
orders of magnitude: a 2 MiB folio split into 512 4 KiB requests can
charge up to 512x its real size, pushing global dirty+writeback
accounting past the system's dirty threshold and forcing every
buffered writer on the host into the hard-throttle path, including
unrelated in-kernel NFS server threads sharing the box.

Charge each request only for the pages it actually covers.

The Linux kernel CVE team has assigned CVE-2026-72132 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.3 with commit 0c493b5cf16e28d761b6e77c7c32aa0e7af70813 and fixed in 6.6.148 with commit a442c258320b689f13d2205eaeeddf8b0e630288
	Issue introduced in 6.3 with commit 0c493b5cf16e28d761b6e77c7c32aa0e7af70813 and fixed in 6.12.97 with commit 1f646e23372f3444dc5f0bcb5404a49d26756add
	Issue introduced in 6.3 with commit 0c493b5cf16e28d761b6e77c7c32aa0e7af70813 and fixed in 6.18.40 with commit 0ffc032294a29601b1019dba91aa1a930d90df17
	Issue introduced in 6.3 with commit 0c493b5cf16e28d761b6e77c7c32aa0e7af70813 and fixed in 7.1.5 with commit a192b6c149c6ea10cc88869accb78165eb454456
	Issue introduced in 6.3 with commit 0c493b5cf16e28d761b6e77c7c32aa0e7af70813 and fixed in 7.2-rc3 with commit 27934d02cbeb8a957dd11c985a579e58d30c5270

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72132
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/nfs/internal.h
	fs/nfs/pnfs_nfs.c
	fs/nfs/write.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a442c258320b689f13d2205eaeeddf8b0e630288
	https://git.kernel.org/stable/c/1f646e23372f3444dc5f0bcb5404a49d26756add
	https://git.kernel.org/stable/c/0ffc032294a29601b1019dba91aa1a930d90df17
	https://git.kernel.org/stable/c/a192b6c149c6ea10cc88869accb78165eb454456
	https://git.kernel.org/stable/c/27934d02cbeb8a957dd11c985a579e58d30c5270