CVE-2026-43389: mm: memfd_luo: always dirty all folios

CVE-2026-43389: mm: memfd_luo: always dirty all folios

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

mm: memfd_luo: always dirty all folios

A dirty folio is one which has been written to.  A clean folio is its
opposite.  Since a clean folio has no user data, it can be freed under
memory pressure.

memfd preservation with LUO saves the flag at preserve().  This is
problematic.  The folio might get dirtied later.  Saving it at freeze()
also doesn't work, since the dirty bit from PTE is normally synced at
unmap and there might still be mappings of the file at freeze().

To see why this is a problem, say a folio is clean at preserve, but gets
dirtied later.  The serialized state of the folio will mark it as clean. 
After retrieve, the next kernel will see the folio as clean and might try
to reclaim it under memory pressure.  This will result in losing user
data.

Mark all folios of the file as dirty, and always set the
MEMFD_LUO_FOLIO_DIRTY flag.  This comes with the side effect of making all
clean folios un-reclaimable.  This is a cost that has to be paid for
participants of live update.  It is not expected to be a common use case
to preserve a lot of clean folios anyway.

Since the value of pfolio->flags is a constant now, drop the flags
variable and set it directly.

The Linux kernel CVE team has assigned CVE-2026-43389 to this issue.


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

	Issue introduced in 6.19 with commit b3749f174d686627f702234e64bad976dc432dbc and fixed in 6.19.9 with commit e901c871d4b592f0042e30f3a0f031eae79744ec
	Issue introduced in 6.19 with commit b3749f174d686627f702234e64bad976dc432dbc and fixed in 7.0 with commit 7e04bf1f33151a30e06a65b74b5f2c19fc2be128

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-2026-43389
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/memfd_luo.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/e901c871d4b592f0042e30f3a0f031eae79744ec
	https://git.kernel.org/stable/c/7e04bf1f33151a30e06a65b74b5f2c19fc2be128