* [CVE-2026-80775][IMPORTANT] futex: Fix race on the initial mm->futex.phash.ref allocation
@ 2026-09-04 16:38 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-04 16:38 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-80775
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: futex: Fix race on the initial mm->futex.phash.ref allocation
Commit: 86d12b34bafc9a4c271a9edefea88855a934d6e5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86d12b34bafc9a4c271a9edefea88855a934d6e5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80775
Analysis date: Fri, 04 Sep 2026 12:38:48 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES
Summary:
A race in futex_hash_allocate() can let two tasks sharing the same mm publish different private futex hash reference counters, causing incorrect lifetime tracking and a use-after-free with a write into freed memory in futex_q_lock(), making this at least a local DoS and a plausible LPE candidate requiring manual review.
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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80775 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80775
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.
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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80775 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-80775 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-362;*CWE-416;CWE-667;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 futex_hash_allocate can allocate and publish the initial mm futex phash ref counter from two tasks that share the same mm. With nested vfork and concurrent PR_FUTEX_HASH operations, both tasks can observe a NULL counter and store different percpu counters, leaving the first counter unreachable from the mm. This can make a private futex hash appear dead while references still exist, leading to a use-after-free and a write into freed memory in futex_q_lock. For the CVSS the PR:L is used because a local unprivileged process can create the required task relationship and call the futex hash prctl interface. The issue is not network reachable. Impact is at least local denial of service via kernel memory corruption, and in the paranoid case may allow privilege escalation due to the freed memory write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) YES LOCK INIT NOMEMCHK RACE UAF HARDWARE LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS YES NO checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
ActionableScoreLower=6
## 1. ActionableScore
* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::
## 2. Signal breakdown
* Local unprivileged trigger: +1. A local user can create the nested vfork / shared mm condition and call prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS).
* Privilege escalation plausible: +1 conservative, +2 paranoid. The patch explicitly describes UAF and a write into freed memory in futex_q_lock(), but does not demonstrate controlled reclaim or a full exploit primitive.
* Generic memory corruption: +2. This is a real UAF with write-after-free behavior, not only a NULL dereference or warning.
* Real lifetime corruption: +1. The lost percpu ref counter causes incorrect lifetime accounting for the private futex hash.
* Reliable kernel crash / strong DoS: +1. Writing into freed kernel memory is a realistic crash path.
* Broad/default/common subsystem: +1. Futex is a core Linux userspace synchronization interface.
* Hard or unreliable race / special timing required: -1. Triggering requires a nested vfork race and concurrent PR_FUTEX_HASH operations.
Paranoid score raises LPE plausibility to +2 because the primitive is a kernel UAF with an actual write into freed memory, which is historically worth manual review even if exploitability is not proven.
## 3. Reachability analysis
The bug is locally reachable and does not require network access. The likely attacker is a local unprivileged user able to run normal processes, create nested vfork relationships, signal tasks, and invoke the futex hash prctl interface. Containers and namespaces may matter because the primitive is tied to process creation and shared mm behavior rather than a privileged device or admin-only interface. The path is in a core kernel synchronization subsystem and is not a rare driver or non-default hardware feature. Realistic exploitation still requires winning a race and shaping lifetime timing, so DoS is much more strongly supported than reliable privilege escalation.
## 4. Severity interpretation
This is stronger than an ordinary Moderate because the patch describes a concrete UAF and a write into freed memory. It is not merely a bookkeeping leak or validation-only fix. Conservative handling is Actionable Moderate because local unprivileged reachability plus kernel memory corruption should not be auto-closed. Paranoid handling is a Strong Important candidate because a writable UAF in a core local API can plausibly become LPE after further analysis, even though no controlled reclaim, object replacement, or public exploit is shown here.
## 5. One-sentence report phrase
A race in futex_hash_allocate() can let two tasks sharing the same mm publish different private futex hash reference counters, causing incorrect lifetime tracking and a use-after-free with a write into freed memory in futex_q_lock(), making this at least a local DoS and a plausible LPE candidate requiring manual review.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch explicitly describes kernel UAF plus write-after-free in a broadly reachable local subsystem. YES REQUIRES MANUAL CHECK.
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: futex: Fix race on the initial mm->futex.phash.ref allocation
Commit: 86d12b34bafc9a4c271a9edefea88855a934d6e5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86d12b34bafc9a4c271a9edefea88855a934d6e5
Commit description:
futex_hash_allocate() allocates mm->futex.phash.ref without any locking.
Commit d9b0532 ("futex: Move futex_hash_free() back to __mmput()")
moved the allocation here and assumed that the process has just a single
thread at this point.
Commit ee9dce4 ("futex: Drop CLONE_THREAD requirement for private
default hash alloc") widened need_futex_hash_allocate_default() to cover
any CLONE_VM clone, but left out vfork because the parent is suspended and
cannot race.
That no longer holds once vfork is nested. If a vfork child calls vfork
again and is then killed with SIGKILL, the parent is released from its
vfork wait and runs concurrently with the grandchild in the same mm.
Neither of them went through futex_hash_allocate_default().
When both call prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS) at the same
time, each one sees mm->futex.phash.ref as NULL and stores its own percpu
counter. Only the last store survives. The counter stored first is no
longer reachable from the mm, so the references on it are not seen by
__futex_ref_atomic_end(). A private hash that still has references is then
considered dead and freed, and a task that still holds one of its buckets
writes into freed memory in futex_q_lock().
Store the counter once with cmpxchg() and let the loser free_percpu() its
own. The initial reference has to be taken before the store, otherwise
another task can install a private hash while the counter is still 0.
Fixes: d9b0532 ("futex: Move futex_hash_free() back to __mmput()")
Signed-off-by: Hyunwoo Kim <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: https://patch.msgid.link/ansrpP4ImE1MaBY9@v4bel
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
kernel/futex/core.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=86d12b34bafc9a4c271a9edefea88855a934d6e5
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80775 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80775
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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