GHSA-j9m4-fr5f-49wm: Type confusion in the device_deinit system call allows user-mode threads to execute arbitrary kernel code

The user-mode verification handler for the devicedeinit() system call, zvrfydevicedeinit() in kernel/device.c, validated its dev argument with KSYSCALLOBJINIT(dev, KOBJANY). kobjectvalidate() short-circuits its type comparison when the requested type is KOBJANY, so the check reduced to "this pointer is the base address of some kernel object the calling thread has been granted" — the object's actual type was never compared, and KSYSCALLOBJINIT also skips the initialization-state check. The sibling handlers zvrfydeviceinit() and zvrfydeviceisready() already used KOBJDRIVERANY and were unaffected.

A thread running in user mode can therefore pass any kernel object it holds permission on — most usefully a thread stack object obtained from the kthreadstackalloc() syscall or a statically defined KTHREADSTACK it was granted in order to spawn a child user thread — whose backing memory is writable from user mode. zimpldevicedeinit() then interprets those attacker-written bytes as a struct device: it dereferences the state pointer read out of the object, calls the function pointer read out of ops.deinit, and on success writes through state again. The result is an indirect call to an arbitrary address executed in supervisor mode, plus an arbitrary kernel read and a single-byte kernel write.

Exploitation gives a local unprivileged thread full kernel code execution, defeating the CONFIGUSERSPACE isolation boundary entirely; a less precise attempt yields a supervisor-mode fault and a system crash. The defect is only reachable in builds that enable both CONFIGUSERSPACE and CONFIGDEVICEDEINITSUPPORT — with de-initialization support disabled, zimpldevicedeinit() returns -ENOTSUP without ever dereferencing the pointer. In v4.2.x and v4.3.x, CONFIGDEVICEDEINITSUPPORT defaulted to y, so every CONFIGUSERSPACE build of those releases is exposed unless the option was explicitly turned off. From v4.4.0 the option is opt-in (no default, and not selected by any in-tree subsystem), so a v4.4.x build is exposed only if it enables the option explicitly. The v4.2 line is no longer maintained and receives no backport.

The fix changes the object check to KOBJDRIVERANY, which constrains the argument to the build-generated driver object type range (KOBJDRIVERFIRST..KOBJDRIVERLAST) — the real struct device instances placed by the linker — so the state and ops.deinit fields are once again kernel-controlled.