UPSTREAM: cgroup/cpuset: Prevent UAF in proc_cpuset_show()

[ Upstream commit 1be59c97 ]

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 d23b5c57 ("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


Bug: 391748468
Fixes: a79a908f ("cgroup: introduce cgroup namespaces")
Change-Id: I5f355cdad6acfc9c51b93b053477cb5920aa077c
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>