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

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/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