[CVE-2026-63802][MODERATE 7.0] blk-cgroup: fix UAF in __blkcg_rstat_flush()

[CVE-2026-63802][MODERATE 7.0] blk-cgroup: fix UAF in __blkcg_rstat_flush()

[AL-KERNEL]

CVE: CVE-2026-63802
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: blk-cgroup: fix UAF in __blkcg_rstat_flush()
Commit: 96e545410c4f74c89d496c1d5d9ef8d08f14368b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=96e545410c4f74c89d496c1d5d9ef8d08f14368b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63802
Analysis date: Sun, 19 Jul 2026 09:16:31 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in blk-cgroup rstat flushing can free a blkcg_gq while another concurrent flush is still iterating over its iostat entries, causing a local race-dependent UAF with realistic DoS impact and limited but review-worthy memory corruption concern.

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

The original Linux kernel CVE announcement for CVE-2026-63802 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63802

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-2026-63802	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-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in blk-cgroup rstat flushing can lead to use-after-free when multiple blkcg_gq objects in the same blkcg are released concurrently. One release path can remove another blkg iostat entries with llist_del_all while the other release path observes an empty list and frees the blkg before the first flush has finished iterating over those entries. For the CVSS the PR:L is used for the paranoid score because reliable triggering may be possible for a local user or service that can create and remove delegated cgroups and generate block I/O, while full host administrator rights are not necessarily required in delegated cgroup environments. The issue is not network reachable. Impact is at least local denial of service via kernel crash, and in the worst case the UAF bug class may allow confidentiality and integrity impact if freed kernel memory is reused in an attacker influenced way.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	INIT RACE UAF KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or service may be able to create/remove delegated cgroups and generate block I/O in some deployments. This is not remote reachable.

* Memory corruption, weak/indirect primitive: +1
  The patch explicitly fixes a UAF, but it is a race-dependent lifetime bug without demonstrated attacker-controlled reclaim, overwrite, type confusion, callback dispatch, or refcount takeover.

* Real lifetime corruption: +1
  The bug is a real object lifetime issue involving blkcg_gq release, RCU callback timing, and concurrent traversal of iostat entries.

* Reliable kernel crash / strong DoS: +1
  A stale object being freed while another path iterates over its entries can plausibly cause kernel crash or memory access failure.

* Important filesystem/storage path: +1
  The affected code is in blk-cgroup and block I/O accounting/stat flushing, which is part of a relevant storage control path.

Negative signals:

* Hard or unreliable race / special timing required: -1
  Triggering requires concurrent release of multiple blkgs in the same blkcg and overlap with __blkcg_rstat_flush() iteration.

Race-UAF cap applied:

* Conservative score capped at 4.
* Paranoid score capped at 5.
* Reason: race-dependent UAF with no demonstrated reclaim, write-after-free, object replacement, type confusion, callback control, or arbitrary write.

## 3. Reachability analysis

The issue is local, not network reachable. The likely trigger requires creating/removing blk-cgroup related objects while generating enough block I/O/stat activity to hit concurrent blkg release and rstat flushing. In strict host configurations this may require privileged cgroup management, but in delegated cgroup environments a local user, service account, or container-adjacent workload may have enough control to exercise the path. Namespaces and cgroup delegation can therefore reduce the practical privilege requirement from full host root to PR:L-like conditions, but this depends on product configuration.

The path is not exotic hardware-specific and belongs to the block/cgroup subsystem, but the race window makes reliable exploitation harder than a simple deterministic syscall crash.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is an explicit kernel UAF in object lifetime handling. Realistically, the demonstrated impact is local DoS through crash or corrupted traversal state. Theoretical LPE concern exists because UAF is a memory corruption class, but the patch does not show attacker-controlled reclaim, stale writable access, type confusion, callback dispatch, or another concrete exploitation primitive. Therefore it is best treated as Actionable Moderate, with manual review required, rather than automatically promoted to Strong Important.

## 5. One-sentence report phrase

A race in blk-cgroup rstat flushing can free a blkcg_gq while another concurrent flush is still iterating over its iostat entries, causing a local race-dependent UAF with realistic DoS impact and limited but review-worthy memory corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an explicit race-based UAF in kernel lifetime management. The current evidence supports DoS more strongly than LPE, but the bug class and cgroup/block lifetime context are important enough that it should not be auto-closed.

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

Patch: blk-cgroup: fix UAF in __blkcg_rstat_flush()
Commit: 96e545410c4f74c89d496c1d5d9ef8d08f14368b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=96e545410c4f74c89d496c1d5d9ef8d08f14368b

Commit description:

When multiple blkgs in the same blkcg are released concurrently,
a use-after-free can occur. The race happens when one blkg's
__blkcg_rstat_flush() removes another blkg's iostat entries via
llist_del_all(). The second blkg sees an empty list and proceeds
to free itself while the first is still iterating over its entries.

Move the flush from __blkg_release() (RCU callback) to blkg_release()
(before call_rcu). This ensures the RCU grace period waits for any
concurrent flush's rcu_read_lock() section to complete before freeing.

Cc: stable@vger.kernel.org
Cc: Jay Shin <jaeshin@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Fixes: 20cb1c2 ("blk-cgroup: Flush stats before releasing blkcg_gq")
Reported-by: coregee2000@gmail.com
Closes: https://lore.kernel.org/linux-block/CAHPqNmwT9oRpem3J3erS_W0uSQND47LGGSBsNxP8E6uSUish1w@mail.gmail.com/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
Link: https://patch.msgid.link/20260205155425.342084-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  block/blk-cgroup.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=96e545410c4f74c89d496c1d5d9ef8d08f14368b

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

The original Linux kernel CVE announcement for CVE-2026-63802 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63802

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:N/I:N/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: 4
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).