Re: [PATCH V3] cgroup/cpuset: Prevent UAF in proc_cpuset_show()

[Tejun Heo <tj@kernel.org>]

On Wed, Jun 26, 2024 at 09:41:01AM +0000, Chen Ridong wrote:
> 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_list is already RCU-safe, css->cgroup is safe.
> This is similar to commit 9067d90006df ("cgroup: Eliminate the
> need for cgroup_mutex in proc_cgroup_show()")
> 
> [1] https://syzkaller.appspot.com/bug?extid=9b1ff7be974a403aa4cd
> 
> Fixes: a79a908fd2b0 ("cgroup: introduce cgroup namespaces")
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Applied to cgroup/for-6.10-fixes w/ stable cc added.

Thanks.

-- 
tejun