CVE-2026-89725: media: cec: stm32: prevent out-of-bounds write on RX overflow

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

media: cec: stm32: prevent out-of-bounds write on RX overflow

stm32_rx_done() appends each received CEC byte to rx_msg.msg[] using
rx_msg.len as the write index, incrementing it on every RXBR
(receive-byte-ready) interrupt without checking it against the buffer
size:

	cec->rx_msg.msg[cec->rx_msg.len++] = val & 0xFF;

rx_msg.msg[] is a fixed CEC_MAX_MSG_SIZE (16) byte array in struct
cec_msg, and rx_msg.len is only reset on RXACKE/RXOVR or after a
completed message (RXEND). The number of bytes received before RXEND is
decided by the remote CEC device (it sets EOM), not by the driver. A
peer that keeps sending bytes without ending the message drives RXBR
repeatedly, pushing rx_msg.len past 16 and writing peer-controlled bytes
out of bounds into the surrounding memory. This is reachable in normal
operation once the driver has probed and receiving is enabled, from the
IRQ thread, without any local privilege.

The length check in the CEC core runs on the consumer side, after the
byte has been stored, so it does not prevent the overflow. Bound the
index in the driver before the store, as the other platform CEC drivers
already do (e.g. tegra_cec), dropping the excess bytes of an overlong
frame.

Found by static analysis tool CodeQL.

The Linux kernel CVE team has assigned CVE-2026-89725 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.13 with commit d69ae57453c80a2193bcdb3fd762efbec447eecf and fixed in 6.12.109 with commit c3edd7fb8e3692423a702705509947fc9c30b3d1
	Issue introduced in 4.13 with commit d69ae57453c80a2193bcdb3fd762efbec447eecf and fixed in 6.18.50 with commit 0c260d3f97e52978a8c769bb095ac0901333a3ac
	Issue introduced in 4.13 with commit d69ae57453c80a2193bcdb3fd762efbec447eecf and fixed in 7.2.4 with commit a85ce3b175615c2e13bc01e8a1b6b6b6910fde57
	Issue introduced in 4.13 with commit d69ae57453c80a2193bcdb3fd762efbec447eecf and fixed in 7.3-rc1 with commit fb9dda38d4b9e90db07ed9a0ee2d35bf85494035

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89725
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/media/cec/platform/stm32/stm32-cec.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c3edd7fb8e3692423a702705509947fc9c30b3d1
	https://git.kernel.org/stable/c/0c260d3f97e52978a8c769bb095ac0901333a3ac
	https://git.kernel.org/stable/c/a85ce3b175615c2e13bc01e8a1b6b6b6910fde57
	https://git.kernel.org/stable/c/fb9dda38d4b9e90db07ed9a0ee2d35bf85494035