Affected: zephyrproject zephyr

The NXP MCUX TRNG entropy driver in drivers/entropy/entropy_mcux_trng.c passed the caller's byte count straight to the vendor SDK routine TRNG_GetRandomData(). On i.MX RT5xx and RT6xx parts the SDK compiles its TRNG_SW_HEALTH_TESTS variant, which always copies whole 32-bit words and draws entropy rounded up to a multiple of 128 bytes. Its "caller buffer is full" guard tests dataSize == 0, so a request whose length is not a multiple of four makes dataSize underflow past zero and the SDK keeps writing into the caller's buffer for the entire extraction: a 1-byte request results in 128 bytes written, and any non-word-multiple length overflows by up to 127 bytes.

entropy_get_entropy() is a syscall, and its verifier in drivers/entropy/entropy_handlers.c validates only the requested length via K_SYSCALL_MEMORY_WRITE(). With CONFIG_USERSPACE enabled, an unprivileged user-mode thread that has merely been granted the entropy device can therefore choose both the destination address and a length such as 1, and cause the kernel to write up to 127 bytes beyond the region it proved it owns. On these Cortex-M33 targets there is no MMU, so user partitions and kernel data share one SRAM and the overflow can land in adjacent kernel state. The same defect is reached from kernel mode by any caller requesting a non-word-multiple length, including getentropy() and, in builds where sys_csrand_get()/sys_rand_get() resolve to the hardware generator, sys_rand8_get() and sys_rand16_get().

Impact is m…