* [CVE-2026-97938][MODERATE 7.0] reboot: fix cad_pid use-after-free race
@ 2026-09-25 14:59 AL-KERNEL
0 siblings, 0 replies; only message in thread
From: AL-KERNEL @ 2026-09-25 14:59 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-97938
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: reboot: fix cad_pid use-after-free race
Commit: ad72e2566643fff7f01666d845fb026898155e1f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad72e2566643fff7f01666d845fb026898155e1f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97938
Analysis date: Fri, 25 Sep 2026 10:59:00 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-97938 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97938
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-97938 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-97938 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.2';DESCR 'A use-after-free race exists in the global cad_pid pointer handling. proc_do_cad_pid can pass a stale struct pid pointer to pid_vnr and kill_cad_pid can pass the same stale pointer to kill_pid while a concurrent sysctl writer replaces cad_pid and drops the final reference to the old pid object. The race can cause a kernel crash or incorrect PID object handling. For the CVSS the PR:H is used because reliably changing the global cad_pid sysctl normally requires administrator privileges in the initial user namespace. The issue is local and is not directly network reachable. The confirmed bug class is use-after-free. Limited confidentiality and integrity impact are retained only for the paranoid score because freed pid memory could be reused before dereference. Privilege escalation is not demonstrated by the available code path and requires manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES WRITE OOB LOCK DANGER INIT RACE UAF KERNEL_PANIC_PLUS_UAF BOOT KPANIC KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap was applied. The score reflects the documented affected configuration where an unprivileged user could access the cad_pid sysctl through user and PID namespaces. A separate prior change moved the sysctl back to the global reboot table and removed that namespace exposure, but did not repair the underlying lifetime race.
2. Signal breakdown
* +2 Real UAF / active lifetime violation. A reader obtains cad_pid without RCU lifetime protection while a concurrent writer can replace the pointer and drop the final reference. The commit directly documents a KASAN-confirmed dereference of freed struct pid memory in pid_vnr.
* +1 Local unprivileged trigger. The supplied commit explicitly states that an unprivileged user could reach the sysctl through user and PID namespaces at the time of reporting.
* +1 Attacker-influenced active-object lifetime manipulation. The same namespace-reachable sysctl provides read and write operations. An attacker can repeatedly issue concurrent accesses that replace cad_pid and invoke proc_do_cad_pid, driving the relevant reader and writer lifetime transition.
* +1 Reliable kernel crash / strong DoS. The supplied KASAN stack demonstrates dereference of freed pid memory through pid_nr_ns and pid_vnr.
Important non-awarded signals:
* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Strong LPE plausibility is not awarded solely because this is a UAF.
* Structured stale-object leverage is not separately awarded. struct pid is an active kernel object, but the supplied evidence establishes stale dereference through pid_vnr and kill_pid, not meaningful consumption of attacker-controlled post-free fields.
* Concrete repeated release or refcount mutation is not established. The writer performs one put_pid on the replaced old_pid. Neither reader performs a second put_pid, refcount decrement, free, or equivalent lifetime mutation through the stale pointer.
* No paranoid repeated-refcount leverage point is awarded because there is no second write-like stale refcount operation.
* Extra Availability is not awarded because the observed KASAN fault and resulting kernel crash are already counted as the single strong DoS signal.
* Confidentiality and integrity points are not awarded. A stale struct pid dereference can be dangerous, but the patch does not establish a disclosure path, unauthorized modification, controlled reuse, or privilege escalation.
* Remote reachability is not established.
3. Reachability analysis
The vulnerable reader is reached through proc_do_cad_pid when the cad_pid sysctl is accessed. The writer path resolves a requested PID, atomically replaces cad_pid, and drops the old PID reference. The supplied commit explicitly states that, when the issue was reported, an unprivileged user could reach this sysctl through user and PID namespaces because it was registered in pid_table.
This is sufficient evidence for below-host-admin reachability in the affected namespace-exposed configuration. The relevant sysctl access and PID namespace operations were reachable from an attacker-created namespace, and the same interface can exercise both the reader and writer sides of the race. NamespaceScopedPrivilege is therefore YES for that vulnerable configuration.
A later separate change moved cad_pid into the global reboot sysctl table and corrected the namespace and permission mismatch. On kernels containing that separate mitigation but lacking this RCU lifetime fix, changing cad_pid normally requires host-administrative access. That later restriction does not erase the documented lower-privilege attack surface in the earlier affected configuration.
The race is concurrent but does not receive a hard-race penalty. An attacker can repeatedly issue concurrent sysctl reads and writes, rather than depending on a one-shot external interrupt or uncontrollable hardware timing.
No special hardware, firmware, boot parameter, device, or network exposure is required.
4. Primitive and severity interpretation
The exact primitive is a real RCU lifetime violation involving an active structured kernel object. cad_pid is a global pointer to struct pid. The writer can replace the pointer and execute put_pid on the old object while a reader has already loaded that object pointer but has not finished using it. The reader then dereferences potentially freed struct pid storage through pid_vnr or passes it to kill_pid.
This is not fixed stale DMA storage and not merely passive metadata. struct pid participates in active process lookup and signal delivery. However, the supplied patch only demonstrates stale reads and active PID handling after lifetime loss. It does not demonstrate controlled replacement of the freed struct pid with attacker-shaped data, a callback path, arbitrary read or write, refcount takeover, credential manipulation, or a concrete escalation chain.
There is no concrete repeated release or refcount operation through the dangling object. The vulnerable writer performs the lifetime-ending put_pid once. The stale readers dereference or consume the pointer, but do not perform a second put_pid or equivalent mutation. This is therefore not the stronger repeated-release lifetime-corruption category.
PrimitiveClass is CLASS_B because the patch establishes a real kernel use-after-free with active-object processing and a repeatable below-host-admin trigger in affected namespace-exposed kernels. ImportantCandidate is YES because meaningful exploitation leverage beyond the demonstrated crash remains unresolved for an unprivileged attacker. AutoDowngradeAllowed is NO because automatically treating this as an ordinary crash-only issue would discard a concrete, repeatable kernel lifetime-corruption primitive.
The demonstrated practical consequence is local kernel crash or DoS. Privilege escalation remains speculative and is not treated as established.
5. One-sentence report phrase
A namespace-reachable cad_pid sysctl race can free a struct pid while proc_do_cad_pid or kill_cad_pid still uses it, causing a local kernel use-after-free and potentially a kernel crash.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A real, KASAN-confirmed use-after-free is reachable by an unprivileged user in the documented namespace-exposed affected configuration, but the available patch does not resolve whether repeated triggering can yield useful post-free object reuse beyond DoS.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: reboot: fix cad_pid use-after-free race
Commit: ad72e2566643fff7f01666d845fb026898155e1f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad72e2566643fff7f01666d845fb026898155e1f
Commit description:
cad_pid is a single kernel-wide struct pid pointer. proc_do_cad_pid()
reads it and passes it to pid_vnr() without protecting the lifetime of
the referenced struct pid. A concurrent writer can replace cad_pid and
drop the final reference to the old struct pid after the reader has
loaded the pointer but before pid_vnr() has finished dereferencing it,
causing a use-after-free.
kill_cad_pid() has the same lifetime race when it passes cad_pid to
kill_pid().
At the time this issue was reported, an unprivileged user could reach the
sysctl through user and PID namespaces because cad_pid was registered in
pid_table[]. Moving cad_pid back to the global reboot sysctl table
corrected that namespace and permission mismatch, but did not fix the
underlying lifetime race.
Fix this by treating cad_pid as an RCU-protected pointer at both read
sites and by waiting for a grace period before dropping the old reference
on the write side.
call_rcu(&old_pid->rcu, ...) cannot be used here because free_pid()
also queues pid->rcu; queueing the same rcu_head twice can corrupt the
RCU callback list.
Original KASAN crash stack:
kernel/pid.c:545 pid_nr_ns() # reads freed pid->level
kernel/pid.c:556 pid_vnr() # calls pid_nr_ns()
kernel/pid.c:775 proc_do_cad_pid() # calls pid_vnr(cad_pid)
Fixes: 9ec5209 ("[PATCH] replace cad_pid by a struct pid")
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/all/[email protected]/
Suggested-by: Mateusz Guzik <[email protected]>
Suggested-by: Bradley Morgan <[email protected]>
Suggested-by: Oleg Nesterov <[email protected]>
Suggested-by: Eric W. Biederman <[email protected]>
Suggested-by: Pavel Tikhomirov <[email protected]>
Cc: [email protected]
Signed-off-by: Cen Zhang (Microsoft) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Reviewed-by: Bradley Morgan <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Reviewed-by: Pavel Tikhomirov <[email protected]>
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
kernel/pid.c
include/linux/sched.h
include/linux/sched/signal.h
init/main.c
kernel/reboot.c
kernel/signal.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=ad72e2566643fff7f01666d845fb026898155e1f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-97938 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97938
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:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 5.2
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: unknown
Paranoid ActionableScore: 5
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2026-09-25 14:59 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2026-09-25 14:59 [CVE-2026-97938][MODERATE 7.0] reboot: fix cad_pid use-after-free race AL-KERNEL
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox