linux-kernel - Re: [PATCH V3] cgroup/cpuset: Prevent UAF in proc_cpuset

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Message-ID: <7cc422b6-8437-455a-9d3f-718b4504ae9a@redhat.com>
Date: Wed, 26 Jun 2024 10:56:07 -0400
From: Waiman Long <longman@...hat.com>
To: Chen Ridong <chenridong@...wei.com>, tj@...nel.org,
 lizefan.x@...edance.com, hannes@...xchg.org, adityakali@...gle.com,
 sergeh@...nel.org, mkoutny@...e.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] cgroup/cpuset: Prevent UAF in proc_cpuset_show()

On 6/26/24 05:41, 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@...wei.com>
> ---
>   kernel/cgroup/cpuset.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index c12b9fdb22a4..7f4536c9ccce 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -21,6 +21,7 @@
>    *  License.  See the file COPYING in the main directory of the Linux
>    *  distribution for more details.
>    */
> +#include "cgroup-internal.h"
>   
>   #include <linux/cpu.h>
>   #include <linux/cpumask.h>
> @@ -5052,8 +5053,15 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
>   		goto out;
>   
>   	css = task_get_css(tsk, cpuset_cgrp_id);
> -	retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
> -				current->nsproxy->cgroup_ns);
> +	rcu_read_lock();
> +	spin_lock_irq(&css_set_lock);
> +	/* In case the root has already been unmounted */
> +	if (css->cgroup)
> +		retval = cgroup_path_ns_locked(css->cgroup, buf, PATH_MAX,
> +					       current->nsproxy->cgroup_ns);
> +
> +	spin_unlock_irq(&css_set_lock);
> +	rcu_read_unlock();
>   	css_put(css);
>   	if (retval == -E2BIG)
>   		retval = -ENAMETOOLONG;
Reviewed-by: Waiman Long <longman@...hat.com>