[CVE-2026-74518][IMPORTANT] mm/hugetlb: fix list corruption in allocate_file_region_entries()

[CVE-2026-74518][IMPORTANT] mm/hugetlb: fix list corruption in allocate_file_region_entries()

[AL-KERNEL]

CVE: CVE-2026-74518
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: mm/hugetlb: fix list corruption in allocate_file_region_entries()
Commit: 62e1c2741a4d923d9854efd5927a6212aad7a187
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62e1c2741a4d923d9854efd5927a6212aad7a187
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74518
Analysis date: Sat, 15 Aug 2026 12:43:55 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in hugetlbfs reservation cache refill can corrupt the `resv_map` region cache list and may lead to kernel BUG or later use-after-free on shared hugetlbfs mappings, making this a local DoS with plausible memory-corruption impact that requires manual review.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74518 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74518

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: IMPORTANT
Manual review required: YES

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74518	IMPORTANT	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-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in hugetlb reserve map cache refill can corrupt the resv_map region_cache list because list_splice leaves the stack local allocated_regions list head linked after entries are moved. A concurrent shared hugetlbfs mapping user can consume cache entries during the unlocked allocation window and force a second loop iteration, causing list_add to operate on a stale list head. With DEBUG_LIST this is detected as list_add corruption and a kernel BUG. Without DEBUG_LIST the corrupted list can contain a kernel stack address and later become a use after free condition. For the CVSS the PR:L is used because reliable triggering requires local code able to exercise shared hugetlbfs mapping and reservation operations, but it does not necessarily require full host administrator privileges in all deployments. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	READ OOB DANGER INIT UAF LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* **Local unprivileged or reduced-privilege trigger: +1**
  Triggering requires local code able to exercise shared hugetlbfs mappings and reservation operations. This may be a QEMU, SPDK, DPDK, or vhost-user service context rather than full host root.

* **Generic memory corruption, strong primitive: +2**
  The commit explicitly describes list corruption and later use-after-free when `CONFIG_DEBUG_LIST` is not enabled.

* **Real lifetime corruption: +1**
  A stale stack-local list head can remain linked into `resv->region_cache`, creating a real stale pointer / UAF lifetime issue.

* **Reliable kernel crash / strong DoS: +1**
  With `CONFIG_DEBUG_LIST`, this trips `list_add` corruption and causes a kernel BUG. The issue was observed as a real host panic.

* **Weak LPE concern: +1**
  UAF and kernel list corruption make privilege escalation worth manual review, but no controlled reclaim, object replacement, arbitrary write, or callback control is demonstrated.

* **Availability impact realistic: +1**
  This is not only a warning. It produced a host panic in a real dense KVM deployment.

* **Hard race / special timing required: -1**
  The bug requires a concurrent shared `resv_map` consumer during the unlocked allocation window.

* **Configuration / deployment specificity: -1**
  Practical triggering depends on hugetlbfs shared mappings and concurrent reservation traffic, commonly in virtualization or vhost-style deployments but not every default system.

Paranoid adjustment:

* The rare/configuration penalty is not applied in the paranoid score because the failure was observed on a realistic dense KVM host.
* The score is raised to **7** because list corruption plus later UAF in mm/hugetlb is a memory lifetime bug in a privileged kernel memory-management path and should not be auto-closed.

## 3. Reachability analysis

The bug is **local**, not network reachable. It is reachable through hugetlbfs shared mapping activity, especially when the same hugetlbfs inode is mapped by multiple local processes and they generate concurrent `region_chg()`, `region_add()`, or `region_del()` activity on the same `resv_map`.

Typical ordinary desktop exposure is limited. Practical exposure is higher on virtualization hosts using huge pages, for example QEMU guest RAM mapped `MAP_SHARED` together with SPDK/DPDK vhost-user components. Containers or reduced-privilege service accounts may matter if hugetlbfs access and hugepage-backed mappings are delegated into that environment.

Call-site confidence: **high**, because the commit message includes the call path and the exact corruption scenario.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS because the patch fixes a real list corruption condition that can become a UAF without debug list checking. Realistic demonstrated impact is host crash / DoS. Theoretical impact includes possible confidentiality or integrity compromise due to kernel memory corruption, but practical LPE evidence is not demonstrated.

Overall this should be treated as **Actionable Moderate at minimum**, with a **Strong Important candidate** paranoid interpretation for manual review.

## 5. One-sentence report phrase

A race in hugetlbfs reservation cache refill can corrupt the `resv_map` region cache list and may lead to kernel BUG or later use-after-free on shared hugetlbfs mappings, making this a local DoS with plausible memory-corruption impact that requires manual review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch explicitly describes kernel list corruption and later use-after-free, and the issue was observed as a real host panic in a KVM/hugetlbfs deployment.

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

Patch: mm/hugetlb: fix list corruption in allocate_file_region_entries()
Commit: 62e1c2741a4d923d9854efd5927a6212aad7a187
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62e1c2741a4d923d9854efd5927a6212aad7a187

Commit description:

allocate_file_region_entries() tops up resv->region_cache with freshly
allocated file_region descriptors.  The allocation uses GFP_KERNEL, so
resv->lock is dropped around it: the new entries are gathered on a
stack-local list head, allocated_regions, and spliced into
resv->region_cache once the lock is re-acquired.

The splice used list_splice(), which moves the entries but does not
re-initialize the source head, so allocated_regions is left pointing at an
entry that now lives on resv->region_cache.  The top-up runs in a while
loop that re-checks the cache deficit after re-acquiring the lock.  For a
shared mapping the resv_map is shared by every mapper of the hugetlbfs
inode, so a concurrent region_chg()/region_add()/region_del() on the same
resv_map can consume cache entries during the unlocked window and force a
second iteration.  That iteration calls list_add() on the stale head and
corrupts the list; with CONFIG_DEBUG_LIST the __list_add_valid() check
trips:

  list_add corruption. next->prev should be prev (ffffc900011ff7f8),
  but was ffff88814c281460. (next=ffff88814c545640).
  kernel BUG at lib/list_debug.c:31!
   allocate_file_region_entries+0x191/0x420
   region_chg+0x267/0x300
   hugetlb_reserve_pages+0x387/0xc80
   hugetlbfs_file_mmap+0x2ce/0x3f0
   mmap_region+0x1348/0x1a80
   do_mmap+0x85e/0xb90
   vm_mmap_pgoff+0x18c/0x330
   ksys_mmap_pgoff+0x2a1/0x3e0
   do_syscall_64+0xd7/0x420

Without CONFIG_DEBUG_LIST the bad list_add() silently links a kernel-stack
address into resv->region_cache, leading to later use-after-free.

This was observed as a real host panic on a dense KVM host where a QEMU
guest-RAM hugetlbfs file was mapped MAP_SHARED by both QEMU and a separate
SPDK/DPDK vhost-user target, generating concurrent region_* traffic on one
shared resv_map.

Use list_splice_init() so the source head is re-initialized empty after
each splice, making the retry loop safe.

Link: https://lore.kernel.org/20260713171456.300518-2-caixiangfeng@bytedance.com
Fixes: d3ec7b6 ("mm/hugetlb: use list_splice to merge two list at once")
Signed-off-by: Xiangfeng Cai <caixiangfeng@bytedance.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  lib/list_debug.c
  mm/hugetlb.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=62e1c2741a4d923d9854efd5927a6212aad7a187

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

The original Linux kernel CVE announcement for CVE-2026-74518 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74518

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).