CVE-2026-93099: fs/resctrl: Fix UAF from worker threads when domains are removed

CVE-2026-93099: fs/resctrl: Fix UAF from worker threads when domains are removed

[Greg Kroah-Hartman]

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

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

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

fs/resctrl: Fix UAF from worker threads when domains are removed

The mbm_handle_overflow() and cqm_handle_limbo() workers read event counters
and may sleep while doing so. They are scheduled via delayed_work embedded in
struct rdt_l3_mon_domain. Architecture allocates and frees these domains from
CPU hotplug callbacks under cpus_write_lock(), and the workers acquire
cpus_read_lock() to keep the domain alive across their access.

A use-after-free can occur when a worker is blocked waiting for
cpus_read_lock() while the hotplug core holds cpus_write_lock(): the
architecture frees the rdt_l3_mon_domain that contains the worker's
work_struct. When the worker unblocks, the container_of() it performs on the
embedded work pointer dereferences freed memory.

Drop cpus_read_lock() from the workers and instead drain pending and in-flight
work synchronously before the architecture can free the domain.  Since
architecture offlines the domain under cpus_write_lock() after it has been
unlinked from the RCU list and a grace period has elapsed, no new work can be
scheduled. The cancel only needs to wait out existing work.  Drop
rdtgroup_mutex during CPU offline around cancel_delayed_work_sync() so that
a worker waiting on the mutex can complete before re-pinning the work on
a different CPU.

When offlining a CPU the architecture may iterate over resources in any order.
For example, the MBA control domain may be offlined before or after
a corresponding L3 monitor domain. Ensure that resctrl fs cancels the workers
no matter what order the architecture offlines the domains.

The Linux kernel CVE team has assigned CVE-2026-93099 to this issue.


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

	Issue introduced in 4.14 with commit 24247aeeabe99eab13b798ccccc2dec066dd6f07 and fixed in 7.2.6 with commit b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
	Issue introduced in 4.14 with commit 24247aeeabe99eab13b798ccccc2dec066dd6f07 and fixed in 7.3-rc1 with commit 2566b5cd6a275c124e8f154fef6e815f92ec8d5c

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-2026-93099
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/resctrl/monitor.c
	fs/resctrl/rdtgroup.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/b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
	https://git.kernel.org/stable/c/2566b5cd6a275c124e8f154fef6e815f92ec8d5c