OP-TEE has a Use-After-Free race in FF-A shared-memory teardown

Summary

OP-TEE is a Trusted Execution Environment (TEE) designed as companion to a non-secure Linux kernel running on Arm; Cortex-A cores using the TrustZone technology. Starting in version 3.16.0 and prior to 4.11.0, a user-after-free (UAF) race condition exists in the shared memory teardown logic of FF-A within OP-TEE SPMC/SP flows. This only applies when OP-TEE is configured as an SPMC for S-EL0 SPs, that is, with `CFG_SECURE_PARTITION=y`. The function `sp_mem_remove()`, responsible for freeing entries in `smem->receivers` and `smem->regions`, fails to acquire the global `sp_mem_lock` before performing the `free()` operations. Concurrently, other code paths, such as `sp_mem_get_receiver()`, iterate over these same lists without holding a lock, or, like `sp_mem_is_shared()`, iterate while holding the lock but are not serialized against the unprotected `free()` in `sp_mem_remove()`. This creates a cross-thread race where a thread iterating the list can acquire a pointer to an entry (e.g., `struct sp_mem_map_region` or `struct sp_mem_receiver`), and then another thread calls `sp_mem_remove()`, freeing the object. When the first thread resumes and dereferences the pointer, it results in a Use-After-Free vulnerability. Version 4.11.0 fixes the issue.