CVE-2026-74635
Received Received - Intake

Out-of-Bounds Read in Linux Kernel fbdev Bitblit

Publication date: 2026-08-22

Last updated on: 2026-08-22

Assigner: kernel.org

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.

CVSS Scores

EPSS Scores

Probability:
Percentile:

Meta Information

Published
2026-08-22
Last Modified
2026-08-22
Generated
2026-08-22
AI Q&A
2026-08-22
EPSS Evaluated
N/A
NVD
CVE-2026-74635
EUVD
EUVD-2026-64572

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel *

Helpful Resources

Exploitability

CWE
CWE Icon
KEV
KEV Icon
CWE ID Description
CWE-UNKNOWN

Attack-Flow Graph

AI Quick Actions

Instant insights powered by AI
Executive Summary

This vulnerability in the Linux kernel involves an out-of-bounds read in the fbdev bitblit subsystem. The issue occurs in the bit_cursor() function when fetching a glyph under the cursor. The function does not properly validate the glyph index, which can exceed the font's glyph count. This happens when the screen buffer contains values larger than the current font's glyph count, such as when switching fonts or writing arbitrary values to /dev/vcsa via vcs_write(). The lack of bounds checking allows an attacker to trigger a global-out-of-bounds read, potentially leading to memory corruption or crashes.

Detection Guidance

This vulnerability is specific to the Linux kernel's fbdev subsystem and may not have direct network detection methods. Monitor kernel logs for KASAN errors related to out-of-bounds reads in bit_cursor() or soft_cursor(). Check for crashes in framebuffer console operations or vcs_write() calls. Use commands like dmesg | grep -i kasan or journalctl -k | grep -i kasan to detect such errors.

Impact Analysis

This vulnerability could allow an attacker with local access to cause a denial-of-service (system crash) or potentially escalate privileges by exploiting the out-of-bounds read. It may also lead to memory corruption, which could be leveraged for further attacks. Systems using the Linux kernel with fbdev enabled are affected, especially those with vulnerable font configurations or exposed /dev/vcsa interfaces.

Mitigation Strategies

Apply the kernel patch that adds bounds checking in bit_cursor() to prevent out-of-bounds glyph index access. Update to a kernel version containing the fix. If immediate patching is not possible, restrict access to /dev/vcsa and disable framebuffer console operations if not required.

Chat Assistant

Ask questions about this CVE
Hi! I’m here to help you understand CVE-2026-74635. Ask me anything about the vulnerability, its impact, or mitigation strategies.
0/70

EPSS Chart