CVE-2026-43486: arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults

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

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

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

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

arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults

contpte_ptep_set_access_flags() compared the gathered ptep_get() value
against the requested entry to detect no-ops. ptep_get() ORs AF/dirty
from all sub-PTEs in the CONT block, so a dirty sibling can make the
target appear already-dirty. When the gathered value matches entry, the
function returns 0 even though the target sub-PTE still has PTE_RDONLY
set in hardware.

For a CPU with FEAT_HAFDBS this gathered view is fine, since hardware may
set AF/dirty on any sub-PTE and CPU TLB behavior is effectively gathered
across the CONT range. But page-table walkers that evaluate each
descriptor individually (e.g. a CPU without DBM support, or an SMMU
without HTTU, or with HA/HD disabled in CD.TCR) can keep faulting on the
unchanged target sub-PTE, causing an infinite fault loop.

Gathering can therefore cause false no-ops when only a sibling has been
updated:
 - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared)
 - read faults:  target still lacks PTE_AF

Fix by checking each sub-PTE against the requested AF/dirty/write state
(the same bits consumed by __ptep_set_access_flags()), using raw
per-PTE values rather than the gathered ptep_get() view, before
returning no-op. Keep using the raw target PTE for the write-bit unfold
decision.

Per Arm ARM (DDI 0487) D8.7.1 ("The Contiguous bit"), any sub-PTE in a CONT
range may become the effective cached translation and software must
maintain consistent attributes across the range.

The Linux kernel CVE team has assigned CVE-2026-43486 to this issue.


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

	Issue introduced in 6.9 with commit 4602e5757bcceb231c3a13c36c373ad4a750eddb and fixed in 6.12.78 with commit 05d239f2c95e66e27e7fb4e99ee07eb56e3e34b0
	Issue introduced in 6.9 with commit 4602e5757bcceb231c3a13c36c373ad4a750eddb and fixed in 6.18.19 with commit 6f92a7a8b48a523f910ef25dd83808710724f59b
	Issue introduced in 6.9 with commit 4602e5757bcceb231c3a13c36c373ad4a750eddb and fixed in 6.19.9 with commit 09d620555e59768776090073a2c59d2bc8506eb3
	Issue introduced in 6.9 with commit 4602e5757bcceb231c3a13c36c373ad4a750eddb and fixed in 7.0 with commit 97c5550b763171dbef61e6239cab372b9f9cd4a2

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-2026-43486
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:
	arch/arm64/mm/contpte.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/05d239f2c95e66e27e7fb4e99ee07eb56e3e34b0
	https://git.kernel.org/stable/c/6f92a7a8b48a523f910ef25dd83808710724f59b
	https://git.kernel.org/stable/c/09d620555e59768776090073a2c59d2bc8506eb3
	https://git.kernel.org/stable/c/97c5550b763171dbef61e6239cab372b9f9cd4a2