CVE-2022-48988

In the Linux kernel, the following vulnerability has been resolved:

memcg: fix possible use-after-free in memcg_write_event_control()

memcg_write_event_control() accesses the dentry->d_name of the specified control fd to route the write call. As a cgroup interface file cant be renamed, its safe to access d_name as long as the specified file is a regular cgroup file. Also, as these cgroup interface files cant be removed before the directory, its safe to access the parent too.

Prior to 347c4a874710 (memcg: remove cgroup_event->cft), there was a call to __file_cft() which verified that the specified file is a regular cgroupfs file before further accesses. The cftype pointer returned from __file_cft() was no longer necessary and the commit inadvertently dropped the file type check with it allowing any file to slip through. With the invarients broken, the d_name and parent accesses can now race against renames and removals of arbitrary files and cause use-after-frees.

Fix the bug by resurrecting the file type check in __file_cft(). Now that cgroupfs is implemented through kernfs, checking the file operations needs to go through a layer of indirection. Instead, lets check the superblock and dentry type.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0ed074317b835caa6c03bcfa8f133365324673dc
• https://git.kernel.org/stable/c/35963b31821920908e397146502066f6b032c917
• https://git.kernel.org/stable/c/4a7ba45b1a435e7097ca0f79a847d0949d0eb088
• https://git.kernel.org/stable/c/aad8bbd17a1d586005feb9226c2e9cfce1432e13
• https://git.kernel.org/stable/c/b77600e26fd48727a95ffd50ba1e937efb548125
• https://git.kernel.org/stable/c/e1ae97624ecf400ea56c238bff23e5cd139df0b8
• https://git.kernel.org/stable/c/f1f7f36cf682fa59db15e2089039a2eeb58ff2ad