[CVE-2026-63810][LOW] block: Avoid mounting the bdev pseudo-filesystem in userspace

[CVE-2026-63810][LOW] block: Avoid mounting the bdev pseudo-filesystem in userspace

[AL-KERNEL]

CVE: CVE-2026-63810
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: block: Avoid mounting the bdev pseudo-filesystem in userspace
Commit: 197971e6ffc0a6356b2ba2b22beb42bc0f7e412d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810
Analysis date: Sun, 19 Jul 2026 10:03:27 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline Ordinary Moderate. Not an Important candidate
Manual review required: NO

Summary:
Userspace could mount the internal bdev pseudo filesystem and trigger a NULL function pointer dereference through VFS file access paths, causing a local kernel crash, typically requiring CAP_SYS_ADMIN or delegated mount privileges.

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

The original Linux kernel CVE announcement for CVE-2026-63810 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810

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: LOW
Manual review required: NO

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

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

CVE-2026-63810	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-284;CWE-668;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'The bdev pseudo filesystem is an internal kernel filesystem that could be registered for userspace mounting. If a local actor mounts it and then performs file access operations, VFS paths such as lookup_open can reach inode operations that are empty for this filesystem and can result in a NULL function pointer dereference at address 0. For the CVSS the PR:L is used in the paranoid interpretation for environments where mount capability is delegated to a reduced capability root, container root, or service account. The base case is PR:H because normal mounting requires administrative privileges. The issue is not network reachable and is triggered through local mount and file access operations. Impact is denial of service via kernel crash, with no supported evidence of information disclosure or privilege escalation on modern systems.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Ordinary Moderate. Not an Important candidate (with actual score 2)	YES	KASAN NULLPTR INIT HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline Ordinary Moderate. Not an Important candidate**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The trace shows a NULL function pointer dereference at RIP 0x0 through VFS open path after userspace mounts the internal bdev pseudo filesystem.
* Broad/default kernel code exposure: +1. The bdev pseudo filesystem exists as part of the core block layer, but the vulnerable operation is not normally reachable without mount privileges.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Mounting this filesystem normally requires CAP_SYS_ADMIN or equivalent mount privileges.

Conservative total: 1.

Paranoid signals:

* Reliable kernel crash / strong DoS: +1.
* Broad/default kernel code exposure: +1.
* Important filesystem/storage path: +1. The issue is in the block layer pseudo filesystem and VFS mount/open interaction.
* No memory corruption escalation bonus. This is a NULL function pointer dereference, not UAF, OOB write, double free, stale pointer reuse, refcount takeover, or page-cache corruption.
* No confidentiality or integrity bonus. The patch and trace support crash only.
* No full CAP_SYS_ADMIN penalty in paranoid interpretation. A reduced-capability root, container root, or delegated mount-management service may make this practically less than full host-root in some environments.

Paranoid total: 3.

## 3. Reachability analysis

The bug is locally triggerable by mounting the bdev pseudo filesystem and then accessing files through paths that reach empty inode operations. In normal deployments this requires CAP_SYS_ADMIN, so a fully unprivileged local user should not be able to trigger it directly.

Namespaces or containers may matter only if mount capability is delegated to a reduced-privilege context. That supports the paranoid PR:L-style interpretation, but not true PR:N or ordinary unprivileged reachability.

The path is not network reachable. There is no packet-processing, remote protocol, firmware-mediated input, or external storage protocol trigger shown.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS in the conservative case, and a borderline manual-review Moderate in the paranoid case if delegated mount capabilities are relevant.

The theoretical risk is limited because the primitive is a NULL function pointer dereference at address 0. Modern kernels normally prevent useful exploitation of NULL calls, and the patch does not indicate attacker-controlled code pointers, object reuse, memory corruption, information disclosure, or privilege escalation.

## 5. One-sentence report phrase

Userspace could mount the internal bdev pseudo filesystem and trigger a NULL function pointer dereference through VFS file access paths, causing a local kernel crash, typically requiring CAP_SYS_ADMIN or delegated mount privileges.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local mount-triggered NULL dereference DoS with no demonstrated memory corruption primitive, no network exposure, and normal CAP_SYS_ADMIN gating.

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

Patch: block: Avoid mounting the bdev pseudo-filesystem in userspace
Commit: 197971e6ffc0a6356b2ba2b22beb42bc0f7e412d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d

Commit description:

The bdev pseudo-filesystem is an internal kernel filesystem with which
userspace should not interfere. Unregister it so that userspace cannot
even attempt to mount it.

This fixes a bug [1] that occurs when attempting to access files,
because the system call move_mount() uses pointers declared in the
inode_operations structure, which for the bdev pseudo-filesystem
are always equal to 0. `inode->i_op = &empty_iops;`

[1]

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor instruction fetch in kernel mode
 #PF: error_code(0x0010) - not-present page
 PGD 23380067 P4D 23380067 PUD 23381067 PMD 0
 Oops: 0010 [#1] PREEMPT SMP KASAN NOPTI
 CPU: 2 PID: 17125 Comm: syz-executor.0 Not tainted 6.1.155-syzkaller-00350-g84221fde2681 #0
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
 RIP: 0010:0x0

 Call Trace:
 <TASK>
 lookup_open.isra.0+0x700/0x1180 fs/namei.c:3460
 open_last_lookups fs/namei.c:3550 [inline]
 path_openat+0x953/0x2700 fs/namei.c:3780
 do_filp_open+0x1c5/0x410 fs/namei.c:3810
 do_sys_openat2+0x171/0x4d0 fs/open.c:1318
 do_sys_open fs/open.c:1334 [inline]
 __do_sys_openat fs/open.c:1350 [inline]
 __se_sys_openat fs/open.c:1345 [inline]
 __x64_sys_openat+0x13c/0x1f0 fs/open.c:1345
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x6e/0xd8

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Link: https://lore.kernel.org/all/20131010004732.GJ13318@ZenIV.linux.org.uk/T/#
Cc: stable@vger.kernel.org
Signed-off-by: Denis Arefev <arefev@swemel.ru>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260521072857.5078-1-arefev@swemel.ru
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/namei.c
  fs/open.c
  arch/x86/entry/common.c
  block/bdev.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=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d

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

The original Linux kernel CVE announcement for CVE-2026-63810 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 3

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).