Subject: CVE-2024-40913: cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2024-40913 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.19 with commit c8383054506c and fixed in 6.1.95 with commit eac51d9daacd
	Issue introduced in 5.19 with commit c8383054506c and fixed in 6.6.35 with commit d2d3eb377a5d
	Issue introduced in 5.19 with commit c8383054506c and fixed in 6.9.6 with commit b9f58cdae6a3
	Issue introduced in 5.19 with commit c8383054506c and fixed in 6.10 with commit 4b4391e77a6b

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-40913
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/cachefiles/ondemand.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/eac51d9daacd61dcc93333ff6a890cf3efc8c1c0
	https://git.kernel.org/stable/c/d2d3eb377a5d081bf2bed177d354a4f59b74da88
	https://git.kernel.org/stable/c/b9f58cdae6a364a3270fd6b6a46e0fd4f7f8ce32
	https://git.kernel.org/stable/c/4b4391e77a6bf24cba2ef1590e113d9b73b11039