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

Overview

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).

Affected

• Linux >= 998f70d1806bb718a7565f350283e4a79c8cbb4b < 0b6680e306397097a97767447368221fac753809
• Linux >= 998f70d1806bb718a7565f350283e4a79c8cbb4b < 8391ee06d08845a0a165b5fe679ba326915166b2
• Linux >= 998f70d1806bb718a7565f350283e4a79c8cbb4b < 47504742cea7878ebd1bf1491bbed923df6b90b1
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · updated 2026-09-14 · vex