CVE-2026-74635 - Vulnerability Details

- fbdev: bitblit: bound-check glyph index in bit_cursor()

Description

In the Linux kernel, the following vulnerability has been resolved:

fbdev: bitblit: bound-check glyph index in bit_cursor()

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 18c4ef4e765a
("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.

Published: 2026-08-22

Score: n/a

EPSS: n/a

KEV: No

Impact:

Action:

Analysis

Impact

In Linux kernel consoles, the function bit_cursor() reads a glyph index from the screen buffer without proper bounds checking. When an arbitrary 16‑bit value is written to /dev/vcsa, the index can exceed the current font’s character count, causing the driver to read past the end of the font data array. This results in an out-of-bounds read within the kernel memory space.

Affected Systems

The issue impacts all Linux kernel builds that include the fbdev console subsystem before the patch that adds bounds checking to bit_cursor(). It can be triggered by writing an arbitrary value to /dev/vcsa or by switching from a 512-glyph font to a 256-glyph font, which may leave stale data in the screen buffer.

Risk and Exploitability

The vulnerability is limited to the console driver. Exploitation requires write access to /dev/vcsa or control over console font selection. The EPSS score is not available, and the flaw is not listed in CISA’s KEV catalog.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`0998a6cb232674408a03e8561dc15aa266b2f53b`	affected	< 250159ace2dc53c1bdad267aa8da51b638748700
`db5c9a162d2f42bcc842b76b3d935dcc050a0eec`	affected	< 46336f476484f36145e5117e72d7b590f47433ee
`c12003bf91fdff381c55ef54fef3e961a5af2545`	affected	< bf750cfeacf4e47ac72dadc7f05839696efb8576
`9ba1a7802ca9a2590cef95b253e6526f4364477f`	affected	< 94134d70abf9273b70499d97d0adc9185ef21091
`901f44227072be60812fe8083e83e1533c04eed1`	affected	< c1e7351767dd30fc574395c82121e4c67b882da3
`18c4ef4e765a798b47980555ed665d78b71aeadf`	affected	< bc9db0d879c655d5dfd8add32fd60f13e65d132c
`18c4ef4e765a798b47980555ed665d78b71aeadf`	affected	< 9ea879862e66e354e616028c31b39aa3eb6d35d8
`18c4ef4e765a798b47980555ed665d78b71aeadf`	affected	< e033cbf3975a8465f879ebd5989dc35b04423a4d
`a10cede006f9614b465cf25609a8753efbfd45cc`	affected	—
`efaf89a75a29b2d179bf4fe63ca62852e93ad620`	affected	—
`5.10.247`	affected	< 5.10.265
`5.15.197`	affected	< 5.15.216
`6.1.159`	affected	< 6.1.183
`6.6.117`	affected	< 6.6.152
`6.12.58`	affected	< 6.12.104
`5.4.302`	affected	< 5.5
`6.17.8`	affected	< 6.18

Linux

affected

Version	Status	Constraints
`6.18`	affected	—
`0`	unaffected	< 6.18
`5.10.265`	unaffected	≤ 5.10.*
`5.15.216`	unaffected	≤ 5.15.*
`6.1.183`	unaffected	≤ 6.1.*
`6.6.152`	unaffected	≤ 6.6.*
`6.12.104`	unaffected	≤ 6.12.*
`6.18.45`	unaffected	≤ 6.18.*
`7.1.9`	unaffected	≤ 7.1.*
`7.2`	unaffected	≤ *

No data.

Vendor Product Confidence Versions

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`[0998a6cb232674408a03e8561dc15aa266b2f53b,250159ace2dc53c1bdad267aa8da51b638748700)`	affected	code_commit	—
`[db5c9a162d2f42bcc842b76b3d935dcc050a0eec,46336f476484f36145e5117e72d7b590f47433ee)`	affected	code_commit	—
`[c12003bf91fdff381c55ef54fef3e961a5af2545,bf750cfeacf4e47ac72dadc7f05839696efb8576)`	affected	code_commit	—
`[9ba1a7802ca9a2590cef95b253e6526f4364477f,94134d70abf9273b70499d97d0adc9185ef21091)`	affected	code_commit	—
`[901f44227072be60812fe8083e83e1533c04eed1,c1e7351767dd30fc574395c82121e4c67b882da3)`	affected	code_commit	—
`[18c4ef4e765a798b47980555ed665d78b71aeadf,bc9db0d879c655d5dfd8add32fd60f13e65d132c)`	affected	code_commit	—
`[18c4ef4e765a798b47980555ed665d78b71aeadf,9ea879862e66e354e616028c31b39aa3eb6d35d8)`	affected	code_commit	—
`[18c4ef4e765a798b47980555ed665d78b71aeadf,e033cbf3975a8465f879ebd5989dc35b04423a4d)`	affected	code_commit	—
`a10cede006f9614b465cf25609a8753efbfd45cc`	affected	code_commit	—
`efaf89a75a29b2d179bf4fe63ca62852e93ad620`	affected	code_commit	—
`[5.10.247,5.10.265)`	affected	semver	—
`[5.15.197,5.15.216)`	affected	semver	—
`[6.1.159,6.1.183)`	affected	semver	—
`[6.6.117,6.6.152)`	affected	semver	—
`[6.12.58,6.12.104)`	affected	semver	—
`[5.4.302,5.5)`	affected	semver	—
`[6.17.8,6.18)`	affected	semver	—

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`6.18`	affected	semver	—
`[0,6.18)`	unaffected	semver	—
`[5.10.265,5.10.*]`	unaffected	generic	—
`[5.15.216,5.15.*]`	unaffected	generic	—
`[6.1.183,6.1.*]`	unaffected	generic	—
`[6.6.152,6.6.*]`	unaffected	generic	—
`[6.12.104,6.12.*]`	unaffected	generic	—
`[6.18.45,6.18.*]`	unaffected	generic	—
`[7.1.9,7.1.*]`	unaffected	generic	—
`[7.2,*]`	unaffected	generic	—

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Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

Upgrade the Linux kernel to a version that contains the commit adding bounds checking to bit_cursor()
Reboot the system or reload the fbdev driver after upgrading to ensure the new code is active
Change permissions on /dev/vcsa to deny write access from untrusted users or disable the console driver if it is unnecessary

Generated by OpenCVE AI on August 22, 2026 at 19:04 UTC.

Tracking

Sign in to view the affected projects.

Advisories

No advisories yet.

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d

History

Sat, 22 Aug 2026 19:30:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-125

Sat, 22 Aug 2026 15:45:00 +0000

Type	Values Removed	Values Added
Description		In the Linux kernel, the following vulnerability has been resolved: fbdev: bitblit: bound-check glyph index in bit_cursor() 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 18c4ef4e765a ("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.
Title		fbdev: bitblit: bound-check glyph index in bit_cursor()
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700 https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091 https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8 https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576 https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3 https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d

Subscriptions

Linux Linux Kernel

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2026-08-22T15:32:15.280Z

Updated: 2026-08-22T15:32:15.280Z

Reserved: 2026-08-15T05:44:03.922Z

Link: CVE-2026-74635

Vulnrichment

No data.

NVD

Status : Received

Published: 2026-08-22T16:16:36.590

Modified: 2026-08-22T16:16:36.590

Link: CVE-2026-74635

Redhat

No data.

OpenCVE Enrichment

Updated: 2026-08-22T19:15:03Z

Weaknesses

CWE-125
Out-of-bounds Read

Impact

Affected Systems

Risk and Exploitability

Tracking

Subscriptions

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