[CVE-2026-53385][LOW] vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write

[CVE-2026-53385][LOW] vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write

[AL-KERNEL]

CVE: CVE-2026-53385
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write
Commit: 43a6281790273c1b0a9ab76609ff0245b968f1e6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43a6281790273c1b0a9ab76609ff0245b968f1e6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53385
Analysis date: Sun, 19 Jul 2026 10:41:24 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-53385 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53385

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-2026-53385	LOW	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:N/I:N/A:H';**CWE-476;CWE-362;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'vc_screen has a race in vcs_write when console_lock is dropped while copying data from userspace. If the virtual console is deallocated before the lock is reacquired, the revalidation path can set vc to NULL and exit the write loop with written data already recorded. The old code then still called vcs_scr_updated with this NULL vc pointer, causing a NULL pointer dereference in the notifier path and a local kernel crash. For the CVSS the PR:L because the paranoid interpretation assumes a local user or low privileged process can access the vcs device or race with console teardown in the target environment. The issue is not network reachable and the impact is denial of service only. No concrete UAF or arbitrary write primitive is supported by this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	READ NULLPTR PRINTF-TTY RACE UAF IMPROVEONLY  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=1
ActionableScoreLower=0

1. **ActionableScore**
* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**

2. **Signal breakdown**
* Reliable kernel crash / strong DoS: +1. The patch fixes a KASAN observed NULL pointer dereference in `vcs_notifier()`, which can cause a local kernel crash.
* Hard or unreliable race / special timing required: -1. The bug requires a race between `vcs_write()` dropping and reacquiring `console_lock` and concurrent virtual console deallocation.
* Requires admin/root or restricted device access in typical deployments: -2 for conservative scoring. Writing to `/dev/vcs*` and racing console teardown is usually not available to arbitrary users in hardened or default server deployments.
* Local unprivileged trigger: +1 only for paranoid scoring. Some systems may expose `vcs` devices to console users or low-privileged local contexts, but this is deployment-dependent.
* No generic memory corruption score. The fixed failure mode is a NULL pointer dereference after revalidation, not a demonstrated UAF reuse, stale object access, overwrite, refcount abuse, callback control, or type confusion.
* No confidentiality or integrity impact. The patch context supports DoS only.

3. **Reachability analysis**
The bug is local and requires access to the virtual console screen device write path plus a concurrent console teardown or deallocation window. It is not network reachable. Namespace or container impact is likely limited because virtual console devices are usually not exposed into ordinary containers. If a local console user or service account can write to `/dev/vcs*`, the practical PR may be lower, but reliable triggering still depends on a race.

Call-site confidence: high. The patch shows the exact call site where `vcs_scr_updated(vc)` is called after `vc` may become NULL.

4. **Severity interpretation**
This behaves like an ordinary Moderate or Low-like local DoS issue, not an Important-class vulnerability. Theoretical concern comes from the surrounding lifetime race, but the demonstrated primitive is a NULL dereference, not exploitable memory corruption. There is no supported path to privilege escalation, information disclosure, or integrity impact from the provided patch.

5. **One-sentence report phrase**
A race in `vc_screen` can leave `vc` NULL after `vcs_write()` revalidation and still call `vcs_scr_updated()`, causing a local NULL pointer dereference and kernel crash under timing-dependent conditions.

6. **Manual review recommendation**
NO MANUAL CHECK NEEDED. The patch supports a timing-dependent local DoS only, with no concrete UAF reuse, write primitive, information leak, or privilege escalation evidence.

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

Patch: vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write
Commit: 43a6281790273c1b0a9ab76609ff0245b968f1e6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43a6281790273c1b0a9ab76609ff0245b968f1e6

Commit description:

A KASAN null-ptr-deref was observed in vcs_notifier():

BUG: KASAN: null-ptr-deref in vcs_notifier+0x98/0x130
Read of size 2 at addr qmp_cmd_name: qmp_capabilities, arguments: {}

The issue is a race condition in vcs_write(). When the console_lock is
temporarily dropped (to copy data from userspace), the vc_data pointer
obtained from vcs_vc() may become stale. After re-acquiring the lock,
vcs_vc() is called again to re-validate the pointer. If the vc has been
deallocated in the meantime, vcs_vc() returns NULL, and the while loop
breaks (with written > 0). However, after the loop, vcs_scr_updated(vc)
is still called with the now-NULL vc pointer, leading to a null pointer
dereference in the notifier chain (vcs_notifier dereferences param->vc).

Fix this by adding a NULL check for vc before calling vcs_scr_updated().

Fixes: 8fb9ea6 ("vc_screen: reload load of struct vc_data pointer in vcs_write() to avoid UAF")
Cc: stable@vger.kernel.org
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://patch.msgid.link/20260604060734.2914976-1-yiyang13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/tty/vt/vc_screen.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=43a6281790273c1b0a9ab76609ff0245b968f1e6

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

The original Linux kernel CVE announcement for CVE-2026-53385 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53385

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0
  Paranoid ActionableScore: 1

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).