CVE-2026-63949

What the bug is

linedisp_display() unconditionally reads msg[count - 1] before checking whether count is zero, so a write of zero bytes to the message sysfs attribute hits msg[-1]: write(fd, "", 0); -> message_store(..., buf, count=0) -> linedisp_display(linedisp, buf, count=0) -> msg[count - 1] == '\n' ; OOB read The kernfs write buffer for that store is a 1-byte allocation (kernfs_fop_write_iter() does kmalloc(len + 1) with len == 0), so msg[-1] is a 1-byte read before the slab object. On a KASAN-enabled kernel this trips an out-of-bounds report and panics; on stock kernels it silently reads adjacent slab data and, if that byte happens to be '\n', the following count-- wraps ssize_t 0 to -1 and is then passed to kmemdup_nul(). linedisp_display() is reached from the message_store() sysfs callback (drivers/auxdisplay/line-display.c message attribute, mode 0644) and from the in-tree initial-message setup with count == -1, so the OOB path is only userspace-triggerable via zero-byte writes; vfs_write() does not short-circuit on count == 0 and kernfs_fop_write_iter() dispatches the store callback regardless. Guard the trailing-newline trim with a count check. The existing if (!count) block then takes the clear-display path unchanged. Affects every auxdisplay driver that registers via linedisp_register() / linedisp_attach(): ht16k33, max6959, img-ascii-lcd, seg-led-gpio.

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

drivers/auxdisplay/line-display.c

What this is

AUXILIARY DISPLAY DRIVERSMAINTAINERS section matched on drivers/auxdisplay/ · Odd Fixes

Kernel module

none to look for. The file would have gone into line_display, and Debian does not compile it into that module, so finding line_display in lsmod would say nothing about this CVE.

Check it on the machine

grep -w CONFIG_LINEDISP /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

Show whether Debian builds this, per release

Debian does not build this code. In 4 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.

• bookworm, trixie, forky, sidn

n

not enabled on any flavour - this release does not build the code

Build switch

CONFIG_LINEDISP

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Signals

Published

2026-07-19 (3 months ago)source

CVSS

no vector published, unrated

Attack vector

not published

EPSS

0.17%, higher than 6.2% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

drivers/auxdisplay

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
5.16	6.1.176	ca5b0781946d
5.16	6.6.143	8776032fe989
5.16	6.12.93	3859960daeb9
5.16	6.18.35	197476b12601
5.16	7.0.12	6ad4f75ef9f3
5.16	7.1	a7511dcd9dd4