CVE-2021-46966: ACPI: custom_method: fix potential use-after-free issue

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

ACPI: custom_method: fix potential use-after-free issue

In cm_write(), buf is always freed when reaching the end of the
function.  If the requested count is less than table.length, the
allocated buffer will be freed but subsequent calls to cm_write() will
still try to access it.

Remove the unconditional kfree(buf) at the end of the function and
set the buf to NULL in the -EINVAL error path to match the rest of
function.

The Linux kernel CVE team has assigned CVE-2021-46966 to this issue.


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

	Issue introduced in 4.4.195 with commit 4bda2b79a9d0 and fixed in 4.4.269 with commit 1d53ca5d1310
	Issue introduced in 4.9.195 with commit 5c12dadcbef8 and fixed in 4.9.269 with commit 8b04d57f30ca
	Issue introduced in 4.14.147 with commit 35b88a10535e and fixed in 4.14.233 with commit 90575d1d9311
	Issue introduced in 4.19.77 with commit e4467fb6ef54 and fixed in 4.19.191 with commit a5b26a2e362f
	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.4.118 with commit 72814a94c38a
	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.10.36 with commit 62dc2440ebb5
	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.11.20 with commit f16737caf41f
	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.12.3 with commit b7a5baaae212
	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.13 with commit e483bb9a991b

Please see https://www.kernel.org or 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-2021-46966
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:
	drivers/acpi/custom_method.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/1d53ca5d131074c925ce38361fb0376d3bf7e394
	https://git.kernel.org/stable/c/8b04d57f30caf76649d0567551589af9a66ca9be
	https://git.kernel.org/stable/c/90575d1d9311b753cf1718f4ce9061ddda7dfd23
	https://git.kernel.org/stable/c/a5b26a2e362f572d87e9fd35435680e557052a17
	https://git.kernel.org/stable/c/72814a94c38a33239793f7622cec6ace1e540c4b
	https://git.kernel.org/stable/c/62dc2440ebb552aa0d7f635e1697e077d9d21203
	https://git.kernel.org/stable/c/f16737caf41fc06cfe6e49048becb09657074d4b
	https://git.kernel.org/stable/c/b7a5baaae212a686ceb812c32fceed79c03c0234
	https://git.kernel.org/stable/c/e483bb9a991bdae29a0caa4b3a6d002c968f94aa