CVE-2026-87038: Time-of-check/time-of-use double fetch in the Zephyr i3c_transfer() syscall handler allows kernel-mode control-flow hijack from user mode

The user-mode verification handler for the i3ctransfer() system call, zvrfyi3ctransfer() in drivers/i3c/i3chandlers.c, validated the caller-supplied struct i3cdevicedesc in place instead of snapshotting it. It confirmed only that the descriptor was readable by the calling thread (KSYSCALLMEMORYREAD()), read target->bus once to check it with KSYSCALLOBJ(), and then passed the caller's live pointer down. The implementation zimpli3ctransfer() re-reads target->bus from that user memory and dispatches through it as DEVICEAPIGET(i3c, target->bus)->i3cxfers(...), so the bus that was validated is not necessarily the bus that is used. DEVICEAPIGET() performs no runtime type check outside CONFIGDEVICEAPIASSERT builds.

On a build with CONFIGUSERSPACE and an I3C controller driver, a user thread that has been granted an I3C device object can exploit this with a second thread in the same memory domain: one thread calls i3ctransfer() while the other rewrites the bus field of the descriptor. The window between the check and the use spans the nummsgs validation, the message-array access check, a stack VLA allocation and a memcpy() of up to 32 struct i3cmsg, so the race is repeatable and can simply be retried until it lands.

When the substitution succeeds, the kernel dereferences an attacker-chosen pointer as a struct device, loads a function pointer from the attacker-controlled api table and calls it in supervisor mode with attacker-influenced arguments. This yields kernel-mode code execution from an unprivileged user thread — a complete escape of the userspace memory-protection sandbox; lesser outcomes are an arbitrary two-level kernel read or a kernel fault. Builds without CONFIGUSERSPACE, or without an I3C driver, are unaffected.

The fix copies the descriptor into a kernel-stack local with kusermodefromcopy(), validates the bus in that copy, and dispatches on the copy, so the validated value is provably the value used.