CVE-2026-80975: mfd: qnap-mcu: keep the reply buffer alive past a command timeout

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

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

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

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

mfd: qnap-mcu: keep the reply buffer alive past a command timeout

qnap_mcu_exec() publishes an on-stack buffer to the receive path:

	unsigned char rx[QNAP_MCU_RX_BUFFER_SIZE];
	...
	reply->data = rx;
	reply->length = length;

and qnap_mcu_receive_buf() writes into it from the serdev receive path,
which runs out of flush_to_ldisc() and is not serialized against
qnap_mcu_exec() at all. bus_lock cannot cover it, because qnap_mcu_exec()
holds that mutex across wait_for_completion_timeout().

On a timeout qnap_mcu_exec() returns with reply->data still pointing at
its own frame. A reply that arrives late, or an unsolicited message from
the MCU, is then written into a stack frame that has been left, corrupting
whatever runs next on that stack. The same applies when qnap_mcu_write()
fails, since that path returns without touching the reply state either.

Move the receive buffer into struct qnap_mcu. It is 37 bytes and the
structure is devm_kzalloc()ed, so it lives as long as the driver, and a
late write lands in memory that is still valid and is reinitialized by the
next command. bus_lock keeps commands from sharing it.

This deliberately does not clear reply->data or reply->length on the
timeout path. Doing so races with qnap_mcu_receive_buf(), which reads both
after its

	if (!reply->length)
		return size;

check: clearing reply->data gives a NULL dereference, and clearing
reply->length alone removes the reply->received == reply->length exit
condition, so the copy loop runs until the uart chunk is consumed and
overruns the buffer. Leaving both set keeps the write bounded by
reply->length, which qnap_mcu_exec() has already checked against
sizeof(mcu->rx).

The Linux kernel CVE team has assigned CVE-2026-80975 to this issue.


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

	Issue introduced in 6.14 with commit 998f70d1806bb718a7565f350283e4a79c8cbb4b and fixed in 6.18.51 with commit 0b6680e306397097a97767447368221fac753809
	Issue introduced in 6.14 with commit 998f70d1806bb718a7565f350283e4a79c8cbb4b and fixed in 7.2.4 with commit 8391ee06d08845a0a165b5fe679ba326915166b2
	Issue introduced in 6.14 with commit 998f70d1806bb718a7565f350283e4a79c8cbb4b and fixed in 7.3-rc1 with commit 47504742cea7878ebd1bf1491bbed923df6b90b1

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-2026-80975
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/mfd/qnap-mcu.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/0b6680e306397097a97767447368221fac753809
	https://git.kernel.org/stable/c/8391ee06d08845a0a165b5fe679ba326915166b2
	https://git.kernel.org/stable/c/47504742cea7878ebd1bf1491bbed923df6b90b1