Subject: CVE-2022-48797: mm: don't try to NUMA-migrate COW pages that have other uses

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

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

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

mm: don't try to NUMA-migrate COW pages that have other uses

Oded Gabbay reports that enabling NUMA balancing causes corruption with
his Gaudi accelerator test load:

 "All the details are in the bug, but the bottom line is that somehow,
  this patch causes corruption when the numa balancing feature is
  enabled AND we don't use process affinity AND we use GUP to pin pages
  so our accelerator can DMA to/from system memory.

  Either disabling numa balancing, using process affinity to bind to
  specific numa-node or reverting this patch causes the bug to
  disappear"

and Oded bisected the issue to commit 09854ba94c6a ("mm: do_wp_page()
simplification").

Now, the NUMA balancing shouldn't actually be changing the writability
of a page, and as such shouldn't matter for COW.  But it appears it
does.  Suspicious.

However, regardless of that, the condition for enabling NUMA faults in
change_pte_range() is nonsensical.  It uses "page_mapcount(page)" to
decide if a COW page should be NUMA-protected or not, and that makes
absolutely no sense.

The number of mappings a page has is irrelevant: not only does GUP get a
reference to a page as in Oded's case, but the other mappings migth be
paged out and the only reference to them would be in the page count.

Since we should never try to NUMA-balance a page that we can't move
anyway due to other references, just fix the code to use 'page_count()'.
Oded confirms that that fixes his issue.

Now, this does imply that something in NUMA balancing ends up changing
page protections (other than the obvious one of making the page
inaccessible to get the NUMA faulting information).  Otherwise the COW
simplification wouldn't matter - since doing the GUP on the page would
make sure it's writable.

The cause of that permission change would be good to figure out too,
since it clearly results in spurious COW events - but fixing the
nonsensical test that just happened to work before is obviously the
CorrectThing(tm) to do regardless.

The Linux kernel CVE team has assigned CVE-2022-48797 to this issue.

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

	Issue introduced in 5.9 with commit 09854ba94c6a and fixed in 5.10.102 with commit 254090925e16
	Issue introduced in 5.9 with commit 09854ba94c6a and fixed in 5.15.25 with commit b3dc4b9d3ca6
	Issue introduced in 5.9 with commit 09854ba94c6a and fixed in 5.16.11 with commit d187eeb02d18
	Issue introduced in 5.9 with commit 09854ba94c6a and fixed in 5.17 with commit 80d47f5de5e3

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-2022-48797
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/mprotect.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/254090925e16abd914c87b4ad1b489440d89c4c3
	https://git.kernel.org/stable/c/b3dc4b9d3ca68b370c4aeab5355007eedf948849
	https://git.kernel.org/stable/c/d187eeb02d18446e5e54ed6bcbf8b47e6551daea
	https://git.kernel.org/stable/c/80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6