[CVE-2026-74674][MODERATE 7.0] mm: fix incorrect flush address in direct page table reclaim

[CVE-2026-74674][MODERATE 7.0] mm: fix incorrect flush address in direct page table reclaim

[AL-KERNEL]

CVE: CVE-2026-74674
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm: fix incorrect flush address in direct page table reclaim
Commit: 0b8ff21cbda8808c86b18f1b0ca2d0025af9a80a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0b8ff21cbda8808c86b18f1b0ca2d0025af9a80a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74674
Analysis date: Sat, 22 Aug 2026 13:23:12 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An incorrect flush address in direct page table reclaim can leave stale paging-structure-cache references to a freed page table, creating a local hard-to-trigger memory-management corruption risk with plausible DoS and possible privilege-escalation impact on affected architectures.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74674 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74674

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74674	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An incorrect flush address in direct page table reclaim can leave stale paging structure cache references to a page table that has already been cleared and freed. The bug occurs because pte_free_tlb receives addr even though addr points past the range covered by the reclaimed PTE table, so architectures that perform address specific paging structure cache invalidation may flush the wrong address. A local user can potentially reach this path through memory unmap or zap activity, but reliable triggering depends on complex timing and architecture specific TLB behavior such as INVPCID or AMD TCE. For the CVSS the PR:L is used because local code execution by an unprivileged user is enough to exercise the relevant memory management paths, while administrative privileges are not inherently required. Impact is at least local denial of service and may extend to confidentiality and integrity compromise because stale page table references after free and reuse are a plausible memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-74674 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB INIT UAF HARDWARE KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Local user memory operations can reach zap and page table reclaim paths without administrative privileges.
* Generic memory corruption, strong corruption primitive: +2. The bug can leave stale paging structure cache references to a freed page table.
* Real lifetime corruption: +1. The affected object is a page table that is freed and may later be reused while stale CPU-side references may remain.
* Weak LPE concern: +1. Page table lifetime bugs are security-sensitive, but the patch does not demonstrate attacker-controlled reuse or a reliable overwrite primitive.
* Reliable kernel crash / strong DoS: +1. Wrong invalidation before freeing page tables can plausibly cause severe kernel or process memory-management failure.
* Broad/default/common subsystem: +1. This is core mm page-table teardown logic, not a rare driver-only path.
* Hard or unreliable timing / special condition: -1. The bad case depends on complex zap_pte_range control flow, lack of accumulated flushes, architecture-specific invalidation behavior, and possible speculative cache fill.

Paranoid additions/changes:

* Strong LPE plausibility instead of weak LPE concern: +1 net increase. A stale reference to a freed and reused page table is a plausible privilege-escalation class because page-table corruption can affect address translation and memory isolation.
* Integrity impact plausible: +1. If reused page-table memory is observed through stale paging-structure-cache state, incorrect mappings may allow memory corruption or unauthorized modification.
* Confidentiality impact plausible: +1. In the worst defensible case, stale page-table state could expose unintended mappings or stale translations.

Call-site confidence: high. The changed call is directly in zap_pte_range and the commit message explains the lifetime and flush-address consequence.

## 3. Reachability analysis

The bug is local, not network reachable. A local unprivileged process can plausibly exercise the relevant mm paths through ordinary mapping and unmapping activity, although reliable triggering is difficult. Namespaces do not materially raise the privilege requirement because the affected code is common per-process memory teardown rather than a privileged control-plane interface.

The path is broad and default because it is in core memory management. Exploitability is architecture and timing dependent. The commit specifically notes x86 INVPCID behavior and AMD EFER.TCE as cases where address-specific invalidation makes the wrong flush address security-relevant.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. It is a page-table lifetime and stale-translation bug in core mm code. Realistic exploitation evidence is limited, and the reliable path may be difficult, so the conservative score stays in Actionable Moderate territory.

The paranoid score reaches Strong Important candidate because stale references to freed and reused page tables are a high-value memory-corruption primitive. Theoretical privilege escalation is plausible even though the patch does not demonstrate a controlled exploit.

## 5. One-sentence report phrase

An incorrect flush address in direct page table reclaim can leave stale paging-structure-cache references to a freed page table, creating a local hard-to-trigger memory-management corruption risk with plausible DoS and possible privilege-escalation impact on affected architectures.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this affects page-table lifetime and stale CPU translation state in core mm code. Even though exploitation appears complex, the primitive is serious enough that it should not be auto-closed.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: mm: fix incorrect flush address in direct page table reclaim
Commit: 0b8ff21cbda8808c86b18f1b0ca2d0025af9a80a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0b8ff21cbda8808c86b18f1b0ca2d0025af9a80a

Commit description:

When zap_pte_range reclaims a page table, it does:

    pte_free_tlb(tlb, pmd_pgtable(pmdval), addr);

and this is unconditionally wrong: if this code executes, addr *always*
points one past the end of the range covered by the table.  The addr
parameter is used to flush the TLB (really the paging-structure-cache)
to drop references to the to-be-freed table, and any architecture that
cares about the parameter will flush the wrong address.  (But they'll
still free the correct page).

I think it's worth contemplating why the kernel works at all.

If we hit the offending line of code, we will first clear the PMD entry
(line 1954, zap_empty_pte_table), then we will issue pending flushes if
force_flush is set (tlb_flush_mmu_tlbonly(tlb)), then we will skip the
retry on line 1979 (phew!), and then we will do the offending
pte_free_tlb call.  *Or* we will clear the PMD entry immediately before
pte_free_tlb (line 1983, zap_pte_table_if_empty).

If we have any pending flushes (i.e. we actually zapped any last-level
entries) at the time we clear the PMD entry, then the flush really ought
to flush all references to the table (Linus certainly seems to think it
will on all architectures [0]).

The condition under which we have no accumulated flushes at the time of
the clear is very complex (the whole zap_pte_range function has absurdly
complex control flow).  If we do hit the bad case, then we will end up
clearing the PMD entry after the last time the range is flushed, and any
CPU is free to cache a reference to the (empty) page table.  If this
happens due to an ordinary read or write, it would segfault, so it would
be rare.  But the cache could be speculatively filled as well.  Then
we'll flush the wrong address and then free and possibly reuse the
table.

On x86, even flushing the wrong address works on non-KPTI Intel systems
because INVLPG flushes *all* paging-structure-caches, not just the ones
for the target address.  But INVPCID does not, and flush_tlb_one_user
will use INVPCID if it's available.  And then we're toast.  AMD systems
are more susceptible: we set the EFER.TCE bit, which makes even INVLPG
only flush the target address.

I think this might fix an issue in ripgrep reported here:
BurntSushi/ripgrep#3494

[0] https://lore.kernel.org/all/CA+55aFzBggoXtNXQeng5d_mRoDnaMBE5Y+URs+PHR67nUpMtaw@mail.gmail.com/T/#u

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Fixes: 4c640eb ("mm: move pte table reclaim code to memory.c")
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: stable@vger.kernel.org
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/memory.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0b8ff21cbda8808c86b18f1b0ca2d0025af9a80a

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74674 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74674

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 6
  Paranoid ActionableScore: 8

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).