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
Description
The user-mode verification handler for the i3c_transfer() system call, z_vrfy_i3c_transfer() in drivers/i3c/i3c_handlers.c, validated the caller-supplied struct i3c_device_desc in place instead of snapshotting it. It confirmed only that the descriptor was readable by the calling thread (K_SYSCALL_MEMORY_READ()), read target->bus once to check it with K_SYSCALL_OBJ(), and then passed the caller's live pointer down. The implementation z_impl_i3c_transfer() re-reads target->bus from that user memory and dispatches through it as DEVICE_API_GET(i3c, target->bus)->i3c_xfers(...), so the bus that was validated is not necessarily the bus that is used. DEVICE_API_GET() performs no runtime type check outside CONFIG_DEVICE_API_ASSERT builds. On a build with CONFIG_USERSPACE 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 i3c_transfer() while the other rewrites the bus field of the descriptor. The window between the check and the use spans the num_msgs validation, the message-array access check, a stack VLA allocation and a memcpy() of up to 32 struct i3c_msg, 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 CONFIG_USERSPACE, or without an I3C driver, are unaffected. The fix copies the descriptor into a kernel-stack local with k_usermode_from_copy(), validates the bus in that copy, and dispatches on the copy, so the validated value is provably the value used.
Affected products
| Vendor | Product | Versions |
|---|---|---|
| zephyrproject | zephyr | 3.2.0 to <4.3.2; 4.4.0 to <4.5.0 |
References
External links from the record. They lead to third-party sites.
- Fix commit
https://github.com/zephyrproject-rtos/zephyr/commit/550437bc9efd7b733f13b9be73e77fd98858c8a2 - GHSA-86xj-2q6x-h82w
https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w