CVE-2024-40913

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

cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

After installing the anonymous fd, we can now see it in userland and close it. However, at this point we may not have gotten the reference count of the cache, but we will put it during colse fd, so this may cause a cache UAF.

So grab the cache reference count before fd_install(). In addition, by kernel convention, fd is taken over by the user land after fd_install(), and the kernel should not call close_fd() after that, i.e., it should call fd_install() after everything is ready, thus fd_install() is called after copy_to_user() succeeds.

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 Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.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/4b4391e77a6bf24cba2ef1590e113d9b73b11039
• https://git.kernel.org/stable/c/b9f58cdae6a364a3270fd6b6a46e0fd4f7f8ce32
• https://git.kernel.org/stable/c/d2d3eb377a5d081bf2bed177d354a4f59b74da88
• https://git.kernel.org/stable/c/eac51d9daacd61dcc93333ff6a890cf3efc8c1c0
• https://git.kernel.org/stable/c/4b4391e77a6bf24cba2ef1590e113d9b73b11039
• https://git.kernel.org/stable/c/b9f58cdae6a364a3270fd6b6a46e0fd4f7f8ce32
• https://git.kernel.org/stable/c/d2d3eb377a5d081bf2bed177d354a4f59b74da88
• https://git.kernel.org/stable/c/eac51d9daacd61dcc93333ff6a890cf3efc8c1c0
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html