Subject: CVE-2021-47461: userfaultfd: fix a race between writeprotect and exit_mmap()

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

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

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

userfaultfd: fix a race between writeprotect and exit_mmap()

A race is possible when a process exits, its VMAs are removed by
exit_mmap() and at the same time userfaultfd_writeprotect() is called.

The race was detected by KASAN on a development kernel, but it appears
to be possible on vanilla kernels as well.

Use mmget_not_zero() to prevent the race as done in other userfaultfd
operations.

The Linux kernel CVE team has assigned CVE-2021-47461 to this issue.

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

	Issue introduced in 5.7 with commit 63b2d4174c4a and fixed in 5.10.76 with commit 3cda4bfffd4f
	Issue introduced in 5.7 with commit 63b2d4174c4a and fixed in 5.14.15 with commit 149958ecd062
	Issue introduced in 5.7 with commit 63b2d4174c4a and fixed in 5.15 with commit cb185d5f1ebf

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-2021-47461
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/userfaultfd.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/3cda4bfffd4f755645577aaa9e96a606657b4525
	https://git.kernel.org/stable/c/149958ecd0627a9f1e9c678c25c665400054cd6a
	https://git.kernel.org/stable/c/cb185d5f1ebf900f4ae3bf84cee212e6dd035aca