CVE-2026-19947

The device multiplexer pseudo-driver in drivers/misc/devmux/devmux.c exposes two system calls, devmux select get() and devmux select set(). When CONFIG USERSPACE is enabled, their user-mode verifiers z vrfy devmux select get() and z vrfy devmux select set() forwarded directly to the kernel implementations without performing any kernel-object validation — no k object access check() and no driver object check. Compounding this, DEVICE DT INST DEFINE() registered devmux instances with a NULL driver API pointer, which causes scripts/build/gen kobject list.py to skip them entirely, so a devmux device had no struct k object and no permission on it could be granted or withheld in the first place.

Any thread running in user mode can reach the defect: device pointers such as DEVICE DT GET(DT NODELABEL(devmux0)) are compile-time constants available to application code, and the syscall accepted them from a thread that had never been granted access to the device. The kernel implementations do validate the pointer against the static devmux instance table and bounds-check the selection index, so this is not a memory-safety issue; the failure is purely one of authorization. Exposure requires CONFIG USERSPACE=y together with the experimental CONFIG DEVMUX=y (which depends on CONFIG DEVICE MUTABLE) and a zephyr,devmux devicetree node.

An unprivileged user-mode thread can read and change which downstream device a multiplexer forwards to, bypassing the userspace object-permission model. Because a devmux forwards the data, configuration and API of the selected device — its documented use being zephyr,console and zephyr,shell-uart switching — an attacker can suppress console, log and shell output, redirect that output to a downstream device it is able to observe, or flip the selection underneath another thread that is legitimately using the multiplexer. The impact is bounded to the devicetree-fixed set of downstream devices; no out-of-bounds access or arbitrary pointer dereference is reachable.

The fix declares subsystem struct devmux driver api and attaches a DEVICE API(devmux, devmux api) instance to each devmux device so that it becomes a tracked K OBJ DRIVER DEVMUX kernel object, and adds K OOPS(devmux inst get(dev) == SIZE MAX) plus K OOPS(k object access check(dev) != 0) to both verifiers.