[CVE-2026-63809][MODERATE 7.0] bpf: use kvfree() for replaced sysctl write buffer

[CVE-2026-63809][MODERATE 7.0] bpf: use kvfree() for replaced sysctl write buffer

[AL-KERNEL]

CVE: CVE-2026-63809
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: use kvfree() for replaced sysctl write buffer
Commit: d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63809
Analysis date: Sun, 19 Jul 2026 10:23:54 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A cgroup BPF sysctl buffer replacement path used `kfree()` on a `kvzalloc()` buffer that may be `vmalloc()` backed, allowing a local configured trigger to cause kernel memory corruption or a kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-63809 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63809

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-2026-63809	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-762;CWE-763;CWE-119;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A memory management mismatch in the cgroup BPF sysctl path can occur because proc_sys_call_handler allocates a temporary sysctl write buffer with kvzalloc, while __cgroup_bpf_run_filter_sysctl used kfree when replacing that buffer. If the allocation falls back to vmalloc, freeing it with kfree can trigger a kernel fault and may corrupt memory. For the CVSS the PR:L is used because reliable triggering requires a local process able to write an affected sysctl and reach the cgroup BPF sysctl replacement path, but it does not necessarily require full host administrative control when such policy is already installed. The issue is not network reachable and requires local execution or a local service path that performs the sysctl write. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	KASAN BPF HARDWARE LINUS TEST  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Generic memory corruption: +2. The patch fixes `kfree()` being used on memory allocated via `kvzalloc()`, which may be `vmalloc()` backed. This is a concrete allocator mismatch, not just a warning.
* Real lifetime/freeing corruption: +1. The wrong free primitive is used during buffer replacement in `__cgroup_bpf_run_filter_sysctl()`.
* Reliable kernel crash / strong DoS: +1. The commit includes a reproduced kernel Oops in `kfree()` under KASAN and failslab.
* Hard or special conditions required: -1. Reliable reproduction requires the sysctl replacement path plus a large or failed allocation causing `kvzalloc()` to fall back to `vmalloc()`.
* BPF privilege gating limits escalation. No unprivileged BPF bonus is applied because cgroup BPF/sysctl attachment is privileged in typical deployments.
* Paranoid uplift: +1. Because this is allocator misuse with possible memory corruption, limited LPE concern is defensible even without a demonstrated reclaim or arbitrary write primitive.

## 3. Reachability analysis

The bug is local, not network reachable. Triggering requires a sysctl write path that reaches a cgroup BPF sysctl hook and exercises buffer replacement. In typical deployments, attaching the relevant BPF program requires privileged capabilities such as `CAP_BPF`, `CAP_SYS_ADMIN`, or equivalent root-controlled setup. A task inside an already configured target cgroup may be closer to `PR:L` than full host root, but reliable triggering still depends on privileged policy and writable sysctl conditions.

Call-site confidence: high, because the patch and stack trace show the allocation/free path and the failing call chain.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because the primitive is a real allocator mismatch with a demonstrated kernel fault. However, it is not clearly Important by itself because there is no demonstrated attacker-controlled reclaim, arbitrary write, type confusion, or privilege escalation exploit. It should be treated as Actionable Moderate at minimum, with paranoid manual-review handling due to kernel memory corruption potential.

## 5. One-sentence report phrase

A cgroup BPF sysctl buffer replacement path used `kfree()` on a `kvzalloc()` buffer that may be `vmalloc()` backed, allowing a local configured trigger to cause kernel memory corruption or a kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete kernel allocator mismatch with demonstrated Oops and possible memory corruption, even though practical exploitation beyond DoS is not proven and is constrained by BPF/sysctl privilege requirements.

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

Patch: bpf: use kvfree() for replaced sysctl write buffer
Commit: d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e

Commit description:

proc_sys_call_handler() allocates its temporary sysctl buffer with
kvzalloc() and passes it to __cgroup_bpf_run_filter_sysctl(). Since
kvzalloc() may fall back to vmalloc() for large allocations, freeing
that buffer with kfree() is wrong and can corrupt memory.

Use kvfree() to safely handle both kmalloc and kvzalloc()/vmalloc
allocations.

The bug was first flagged by an experimental analysis tool we are
developing for kernel memory-management bugs while analyzing
v6.13-rc1. The tool is still under development and is not yet publicly
available. Manual inspection confirms that the bug is still
present in v7.1-rc5.

Reproduced the bug based on v7.1-rc4 in a QEMU x86_64 guest booted with
KASAN and CONFIG_FAILSLAB enabled. To exercise the replacement path, the
test tree also included the accompanying fix for the stale ret == 1
check in __cgroup_bpf_run_filter_sysctl(). The reproducer confines
failslab injections to the proc_sys_call_handler() range, uses
stacktrace-depth=32, and injects fail-nth=1 while writing 8191 bytes to
/proc/sys/kernel/domainname from a task in the target cgroup. Under
that setup, fail-nth=1 triggered the fault:

  BUG: unable to handle page fault for address: ffffeb0200024d48
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: Oops: 0000  SMP KASAN NOPTI
  CPU: 2 UID: 0 PID: 209 Comm: repro_proc_sys_ Not tainted 7.1.0-rc4-00686-g97625979a5d4  PREEMPT(lazy)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
  RIP: 0010:kfree+0x6e/0x510
  ...
  Call Trace:
   <TASK>
   ? __cgroup_bpf_run_filter_sysctl+0x626/0xc30
   __cgroup_bpf_run_filter_sysctl+0x74d/0xc30
   ? __pfx___cgroup_bpf_run_filter_sysctl+0x10/0x10
   ? srso_return_thunk+0x5/0x5f
   ? __kvmalloc_node_noprof+0x345/0x870
   ? proc_sys_call_handler+0x250/0x480
   ? srso_return_thunk+0x5/0x5f
   proc_sys_call_handler+0x3a2/0x480
   ? __pfx_proc_sys_call_handler+0x10/0x10
   ? srso_return_thunk+0x5/0x5f
   ? selinux_file_permission+0x39f/0x500
   ? srso_return_thunk+0x5/0x5f
   ? lock_is_held_type+0x9e/0x120
   vfs_write+0x98e/0x1000
   ...
   </TASK>

With this fix applied on top of the same test setup, rerunning the
reproducer with fail-nth=1 yields no corresponding Oops reports.

Fixes: 4508943 ("proc: use kvzalloc for our kernel buffer")
Cc: stable@vger.kernel.org

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: Dawei Feng <dawei.feng@seu.edu.cn>
Link: https://lore.kernel.org/r/20260603105317.944304-3-dawei.feng@seu.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/bpf/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=d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e

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

The original Linux kernel CVE announcement for CVE-2026-63809 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63809

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: 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).