Subject: CVE-2021-47491: mm: khugepaged: skip huge page collapse for special files

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

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

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

mm: khugepaged: skip huge page collapse for special files

The read-only THP for filesystems will collapse THP for files opened
readonly and mapped with VM_EXEC.  The intended usecase is to avoid TLB
misses for large text segments.  But it doesn't restrict the file types
so a THP could be collapsed for a non-regular file, for example, block
device, if it is opened readonly and mapped with EXEC permission.  This
may cause bugs, like [1] and [2].

This is definitely not the intended usecase, so just collapse THP for
regular files in order to close the attack surface.

[[email-masked]: fix vm_file check [3]]

The Linux kernel CVE team has assigned CVE-2021-47491 to this issue.

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

	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.10.78 with commit 6d67b2a73b8e
	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.14.16 with commit 5fcb6fce74ff
	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.15 with commit a4aeaa06d45e

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-2021-47491
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:
	mm/khugepaged.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/6d67b2a73b8e3a079c355bab3c1aef7d85a044b8
	https://git.kernel.org/stable/c/5fcb6fce74ffa614d964667110cf1a516c48c6d9
	https://git.kernel.org/stable/c/a4aeaa06d45e90f9b279f0b09de84bd00006e733