CVE-2024-43853 Information

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.$cat /proc//cpuset repeatly. 3.$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//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 (## Reference 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