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

Summary

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).

Timeline

1. Sep 11, 2026, 7:42 PM UTCCVE published
2. Oct 19, 2026, 8:45 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 998f70d1806bb718a7565f350283e4a79c8cbb4b · fixed in 0b6680e306397097a97767447368221fac753809; From 998f70d1806bb718a7565f350283e4a79c8cbb4b · fixed in 8391ee06d08845a0a165b5fe679ba326915166b2; From 998f70d1806bb718a7565f350283e4a79c8cbb4b · fixed in 47504742cea7878ebd1bf1491bbed923df6b90b1

• Linux Linux

  6.14

• KernelLinux

  From 6.14.0 · fixed in 6.18.51; From 6.19.0 · fixed in 7.2.4

References 3