CVE-2026-98838
Received Received - Intake

Heap-Based Buffer Overflow in Linux Kernel Framebuffer Console

Publication date: 2026-10-18

Last updated on: 2026-10-18

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: vt: skip screen update for DEC alignment test on backgroup consoles [BUG] Recently, we encountered a KASAN warning as follows: BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0x11f/0x140 Read of size 1 at addr ff1100015fd9f6a4 by task tty_fbcon_oob/1239 CPU: 7 UID: 0 PID: 1239 Comm: tty_fbcon_oob Not tainted 7.3.0-rc1 #100 PREEMPT(full) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 Call Trace: <TASK> ... kasan_report+0xf0/0x120 fb_pad_aligned_buffer+0x11f/0x140 ccw_putcs+0x86c/0xa80 fbcon_putcs+0x338/0x410 do_update_region+0x21d/0x450 do_con_write+0x1e0e/0x4880 con_write+0x13/0x80 n_tty_write+0x374/0x1010 file_tty_write.isra.0+0x404/0x7a0 ... reproduce: 1) open /dev/tty0, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 8 and op.height = 16 (visible VC1) 2) open /dev/tty1, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 28 and op.height = 24 (invisible VC2) 3) echo 3 > /sys/devices/virtual/graphics/fbcon/rotate_all 4) write EShash8(esc hash8) to tty1 [CAUSE] All VCs render to the framebuffer. setfont only modifies the target VC's font without resizing the framebuffer backing buffer (par->rotated.buf); the buffer is only resized for the visible VC (fbcon_do_set_font -> ... -> vc_do_resize -> update_screen). This relies on the con_should_update() check performed before every update_region(). However, the ESC # 8 path (do_con_trol -> do_update_region) omits the con_should_update() check. After changing the font size of an invisible VC, do_update_region() fills using the new font size against a buffer that was never resized, causing an out-of-bounds access. [FIX] Only push the update when the console is visible and not blanked, add the con_should_update() check in the do_con_trol() like every other call site of do_update_region() (update_region(), invert_screen(), ...).

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Meta Information

Published
2026-10-18
Last Modified
2026-10-18
Generated
2026-10-18
AI Q&A
2026-10-18
EPSS Evaluated
N/A
NVD
CVE-2026-98838
EUVD
EUVD-2026-103778

Affected Vendors & Products

Showing 5 associated CPEs
Vendor Product Version / Range
Linux Linux 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Linux Linux 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Linux Linux 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Linux Linux 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Linux Linux 2.6.12

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Exploitability

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KEV
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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This vulnerability in the Linux kernel involves an out-of-bounds memory access in the framebuffer (fb) subsystem. It occurs when a font size is changed for an invisible virtual console (VC), causing the system to write data beyond the allocated buffer during screen updates. The issue stems from missing a safety check before updating the screen for certain control sequences like ESC # 8.

Detection Guidance

This vulnerability is specific to the Linux kernel's framebuffer console (fbcon) and requires local access to trigger. Detection involves checking kernel logs for KASAN slab-out-of-bounds errors related to fb_pad_aligned_buffer. Monitor dmesg or /var/log/kern.log for errors like 'BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer'.

Impact Analysis

This vulnerability could lead to system crashes, data corruption, or potential privilege escalation if exploited. An attacker with local access might trigger a kernel panic or memory corruption by manipulating font settings on invisible consoles, affecting system stability and security.

Mitigation Strategies

Apply the kernel patch that adds the con_should_update() check in do_con_trol(). Until patched, avoid using ESC #8 sequences or font operations on invisible virtual consoles. Monitor vendor advisories for updates.

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