Subject: CVE-2024-43853: cgroup/cpuset: Prevent UAF in proc_cpuset_show()

From: Greg Kroah-Hartman <[email-masked]>

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

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

cgroup/cpuset: Prevent UAF in proc_cpuset_show()

An UAF can happen when /proc/cpuset is read as reported in [1].

This can be reproduced by the following methods:
1.add an mdelay(1000) before acquiring the cgroup_lock In the
 cgroup_path_ns function.
2.$cat /proc/<pid>/cpuset   repeatly.
3.$mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset/
$umount /sys/fs/cgroup/cpuset/   repeatly.

The race that cause this bug can be shown as below:

(umount)		|	(cat /proc/<pid>/cpuset)
css_release		|	proc_cpuset_show
css_release_work_fn	|	css = task_get_css(tsk, cpuset_cgrp_id);
css_free_rwork_fn	|	cgroup_path_ns(css->cgroup, ...);
cgroup_destroy_root	|	mutex_lock(&cgroup_mutex);
rebind_subsystems	|
cgroup_free_root 	|
			|	// cgrp was freed, UAF
			|	cgroup_path_ns_locked(cgrp,..);

When the cpuset is initialized, the root node top_cpuset.css.cgrp
will point to &cgrp_dfl_root.cgrp. In cgroup v1, the mount operation will
allocate cgroup_root, and top_cpuset.css.cgrp will point to the allocated
&cgroup_root.cgrp. When the umount operation is executed,
top_cpuset.css.cgrp will be rebound to &cgrp_dfl_root.cgrp.

The problem is that when rebinding to cgrp_dfl_root, there are cases
where the cgroup_root allocated by setting up the root for cgroup v1
is cached. This could lead to a Use-After-Free (UAF) if it is
subsequently freed. The descendant cgroups of cgroup v1 can only be
freed after the css is released. However, the css of the root will never
be released, yet the cgroup_root should be freed when it is unmounted.
This means that obtaining a reference to the css of the root does
not guarantee that css.cgrp->root will not be freed.

Fix this problem by using rcu_read_lock in proc_cpuset_show().
As cgroup_root is kfree_rcu after commit d23b5c577715
("cgroup: Make operations on the cgroup root_list RCU safe"),
css->cgroup won't be freed during the critical section.
To call cgroup_path_ns_locked, css_set_lock is needed, so it is safe to
replace task_get_css with task_css.

[1] https://syzkaller.appspot.com/bug?extid=9b1ff7be974a403aa4cd

The Linux kernel CVE team has assigned CVE-2024-43853 to this issue.

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

	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 4.19.321 with commit 27d6dbdc6485
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 5.4.283 with commit 10aeaa47e4aa
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 5.10.225 with commit 688325078a8b
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 5.15.166 with commit 4e8d6ac8fc9f
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 6.1.103 with commit 29a8d4e02fd4
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 6.6.44 with commit 96226fbed566
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 6.10.3 with commit 29ac1d238b3b
	Issue introduced in 4.6 with commit a79a908fd2b0 and fixed in 6.11 with commit 1be59c97c83c

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-2024-43853
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:
	kernel/cgroup/cpuset.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/27d6dbdc6485d68075a0ebf8544d6425c1ed84bb
	https://git.kernel.org/stable/c/10aeaa47e4aa2432f29b3e5376df96d7dac5537a
	https://git.kernel.org/stable/c/688325078a8b5badd6e07ae22b27cd04e9947aec
	https://git.kernel.org/stable/c/4e8d6ac8fc9f843e940ab7389db8136634e07989
	https://git.kernel.org/stable/c/29a8d4e02fd4840028c38ceb1536cc8f82a257d4
	https://git.kernel.org/stable/c/96226fbed566f3f686f53a489a29846f2d538080
	https://git.kernel.org/stable/c/29ac1d238b3bf126af36037df80d7ecc4822341e
	https://git.kernel.org/stable/c/1be59c97c83ccd67a519d8a49486b3a8a73ca28a