CVE-2026-98538: writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes

cleanup_offline_cgwb() prepares at most WB_MAX_INODES_PER_ISW inodes
per call and is called again until the dying wb is drained, but every
call walks wb->b_attached and then wb->b_dirty_time from the same end.
Inodes already prepared (they stay on the list with I_WB_SWITCH set
until the switch worker runs) and inodes that cannot be switched
(I_FREEING, I_WILL_FREE, !SB_ACTIVE, DAX, already on the target wb)
stay where they are, so each pass rescans a growing run of them under
wb->list_lock and a full drain is quadratic in the number of inodes on
the list. With ~17M inodes attached to one dying cgwb we saw this end
in soft lockups, with CPUs reported stuck for 21-48s.

Walk both lists from the oldest end and move every scanned inode to
the newest end, so the next pass starts where the previous one stopped
and the drain becomes linear. b_attached is unordered, so nobody sees
the reorder there. b_dirty_time is ordered by dirtied_when, but the
oldest unscanned inode stays at the end move_expired_inodes() picks
from, sync takes the whole list regardless of order, and prepared
inodes leave the list as soon as the switch work runs and get a new
dirtied_time_when on the new wb anyway, so the only inodes left out of
order are the ones that can never switch (DAX), and only on the dying
wb.

The Linux kernel CVE team has assigned CVE-2026-98538 to this issue.


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

	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 6.1.189 with commit 2d1da58c79b66646721a644c6eb4dd1f9c5c9327
	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 6.6.158 with commit c96b4d1628868e1195da1a896f79e06f350a3187
	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 6.12.112 with commit 624dd0931162523d70f9db8c120b5ec6429a9b97
	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 6.18.55 with commit dd4b92067238771b604b59fdda8efed6d70ee823
	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 7.2.9 with commit 9dac137bf77e0683778fb34c86e730c6129e8cb6
	Issue introduced in 5.14 with commit c22d70a162d3cc177282c4487be4d54876ca55c8 and fixed in 7.3-rc5 with commit f6988c90671e83db79df1b7b9d6fdb0e5947fd84

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-2026-98538
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/fs-writeback.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/2d1da58c79b66646721a644c6eb4dd1f9c5c9327
	https://git.kernel.org/stable/c/c96b4d1628868e1195da1a896f79e06f350a3187
	https://git.kernel.org/stable/c/624dd0931162523d70f9db8c120b5ec6429a9b97
	https://git.kernel.org/stable/c/dd4b92067238771b604b59fdda8efed6d70ee823
	https://git.kernel.org/stable/c/9dac137bf77e0683778fb34c86e730c6129e8cb6
	https://git.kernel.org/stable/c/f6988c90671e83db79df1b7b9d6fdb0e5947fd84