[CVE-2026-74635][MODERATE REGULAR] fbdev: bitblit: bound-check glyph index in bit_cursor()

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-74635
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fbdev: bitblit: bound-check glyph index in bit_cursor()
Commit: 250159ace2dc53c1bdad267aa8da51b638748700
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=250159ace2dc53c1bdad267aa8da51b638748700
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74635
Analysis date: Sat, 22 Aug 2026 16:20:41 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local out-of-bounds read in fbdev `bit_cursor()` allows a crafted or stale console glyph index to read past the active font table, causing a kernel diagnostic crash and possibly limited data exposure through cursor rendering.

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

The original Linux kernel CVE announcement for CVE-2026-74635 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74635

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 REGULAR
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-2026-74635	MODERATE	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:L/I:N/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.1';DESCR 'An out of bounds read in fbdev bit_cursor can occur because the glyph index read from the console screen buffer is masked but not checked against the current font charcount before indexing vc_font.data. A local actor with access to write console screen data through vcs_write or a stale screen buffer after a font switch can cause the cursor path to read past the active font glyph table. For the CVSS the PR:L is used because a local user or local process with access to the relevant console device is required, but full administrator privileges are not inherently required in the vulnerable code path. The issue is not network reachable. Impact is mainly denial of service via kernel fault or diagnostic panic, with possible limited confidentiality impact if out of bounds font adjacent data is rendered through the cursor path. For the paranoid score, this keeps the same OOB read class and uses C:L/I:N/A:H because a limited kernel memory disclosure or crash is plausible, but no write primitive or privilege escalation path is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	READ KASAN OOB LINUS SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Syzkaller reaches the bug through `vcs_write()` to `/dev/vcsa`, so a local process with access to the relevant virtual console device can influence the glyph value.
* Reliable kernel crash / strong DoS: +1. KASAN reports a global out-of-bounds read in `soft_cursor()`, and diagnostic or hardened kernels may turn this into a crash or panic.
* Broad/default/common subsystem exposure: +1. fbcon, vt, and `/dev/vcs*` paths are longstanding local console interfaces, though exposure depends on device permissions and whether virtual consoles are enabled.

Paranoid additional signal:

* Confidentiality impact plausible: +1. The primitive is read-only OOB from font-adjacent kernel data and may be partially observable through cursor rendering, but there is no strong direct leak channel shown.

Signals not awarded:

* No remote reachable signal. The trigger is local console or VCSA access, not packet or network input.
* No generic strong memory corruption. This is an OOB read, not UAF, double free, OOB write, arbitrary write, or type confusion.
* No privilege escalation plausible signal. The patch does not show attacker-controlled overwrite, reclaim, callback control, or object replacement.
* No integrity impact. The vulnerable operation reads past `vc_font.data` but does not write out of bounds.

## 3. Reachability analysis

A local process can potentially trigger the issue by writing crafted 16-bit screen-buffer values through `/dev/vcsa` or by relying on stale high-bit glyph values after changing from a 512-glyph font to a 256-glyph font. Practical reachability depends on permissions for `/dev/vcs*` or `/dev/vcsa*`, active console ownership, and distribution configuration. Containers usually do not expose real virtual console devices by default, so container reachability is limited unless these devices are explicitly passed through.

Call-site confidence: high. The commit message gives the concrete call path from `vcs_write()` to `bit_cursor()` and the patch directly fixes the unchecked glyph index before indexing `vc_font.data`.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class issue. The concrete primitive is a local out-of-bounds read in kernel code with plausible DoS and weak limited disclosure concern. Realistic exploitation evidence supports crash or diagnostic panic more strongly than privilege escalation. Theoretical memory corruption potential is low because the patch shows no write primitive, no lifetime corruption, no attacker-controlled object reuse, and no cross-boundary ownership violation.

## 5. One-sentence report phrase

A local out-of-bounds read in fbdev `bit_cursor()` allows a crafted or stale console glyph index to read past the active font table, causing a kernel diagnostic crash and possibly limited data exposure through cursor rendering.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is not just a resource leak or validation-only cleanup. It is a concrete kernel OOB read reachable from a local console data path, with plausible DoS and limited confidentiality impact, but without evidence for write corruption or LPE.

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

Patch: fbdev: bitblit: bound-check glyph index in bit_cursor()
Commit: 250159ace2dc53c1bdad267aa8da51b638748700
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=250159ace2dc53c1bdad267aa8da51b638748700

Commit description:

bit_cursor() fetches the glyph under the cursor with

	c = scr_readw(vc_pos);
	src = vc_font.data + ((c & charmask) * w * height);

where charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer
value comes directly from scr_readw() and may be larger than the current
font's glyph count.

Syzkaller triggers this via vcs_write(). The Call Trace shows
vcs_write() in vc_screen.c writing an arbitrary 16-bit value with
writev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via
vcs_scr_writew() without checking charcount. The stored value is later
read in bit_cursor() in bitblit.c.

When the font is changed from a font with 512 glyphs to a font with
256 glyphs, the screen buffer can retain characters with the high
bit set from the previous mode, which could also produce the same
out-of-bounds access.

  BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70
  Read of size 16 at addr ffff800086c57970

  Call Trace:
   soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70
   bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365
   fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427
   hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883
   update_region+0x100/0x18c drivers/tty/vt/vt.c:669
   vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685

bit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph
index to vc_font.charcount. Apply the same clamp in bit_cursor() after
extracting the attribute and masking, before indexing fontdata.

The fix completes the bounds checking started in commit 18c4ef4
("fbdev: bitblit: bound-check glyph index in bit_putcs*"), which missed
the cursor path.

This change should be safe because the clamp reuses the existing
contract from fbcon: charcount is maintained under console_lock in
con_font_set() and fbcon_font_set(), and hi_font_mask is cleared when
switching from 512 to 256 glyphs. When stale screen data with high bits
remains after a font switch, or when vcs_write() stores an arbitrary
value, clamping the index to 0 prevents the out-of-bounds read without
changing cursor semantics — the same fallback bit_putcs uses.

Reported-by: syzbot+61b1db46218109869c14@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=61b1db46218109869c14
Link: https://lore.kernel.org/all/6a75205c.01d0871a.3a0d52.0032.GAE@google.com/
Fixes: 18c4ef4 ("fbdev: bitblit: bound-check glyph index in bit_putcs*")
Cc: stable@vger.kernel.org
Assisted-by: Hermes:muse-spark-1.2 syzkaller
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/video/fbdev/core/softcursor.c
  drivers/video/fbdev/core/bitblit.c
  drivers/video/fbdev/core/fbcon.c
  drivers/tty/vt/vt.c
  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=250159ace2dc53c1bdad267aa8da51b638748700

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

The original Linux kernel CVE announcement for CVE-2026-74635 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74635

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:L/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).