CVE-2021-46937: mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()'

CVE-2021-46937: mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()'

[Greg Kroah-Hartman]

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

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

mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()'

DAMON debugfs interface increases the reference counts of 'struct pid's
for targets from the 'target_ids' file write callback
('dbgfs_target_ids_write()'), but decreases the counts only in DAMON
monitoring termination callback ('dbgfs_before_terminate()').

Therefore, when 'target_ids' file is repeatedly written without DAMON
monitoring start/termination, the reference count is not decreased and
therefore memory for the 'struct pid' cannot be freed.  This commit
fixes this issue by decreasing the reference counts when 'target_ids' is
written.

The Linux kernel CVE team has assigned CVE-2021-46937 to this issue.


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

	Issue introduced in 5.15 with commit 4bc05954d007 and fixed in 5.15.13 with commit ffe4a1ba1a82
	Issue introduced in 5.15 with commit 4bc05954d007 and fixed in 5.16 with commit ebb3f994dd92

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-2021-46937
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/damon/dbgfs.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/ffe4a1ba1a82c416a6b3a09d46594f6a885ae141
	https://git.kernel.org/stable/c/ebb3f994dd92f8fb4d70c7541091216c1e10cb71