CVE-2024-43853

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 (cgroup: Make operations on the cgroup root_list RCU safe), css->cgroup wont 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

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

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