CVE-2023-52503: Use After Free

In the Linux kernel, the following vulnerability has been resolved:

tee: amdtee: fix use-after-free vulnerability in amdtee_close_session

There is a potential race condition in amdtee_close_session that may cause use-after-free in amdtee_open_session. For instance, if a session has refcount == 1, and one thread tries to free this session via:

kref_put(&sess->refcount, destroy_session);

the reference count will get decremented, and the next step would be to call destroy_session(). However, if in another thread, amdtee_open_session() is called before destroy_session() has completed execution, alloc_session() may return 'sess' that will be freed up later in destroy_session() leading to use-after-free in amdtee_open_session.

To fix this issue, treat decrement of sess->refcount and removal of 'sess' from session list in destroy_session() as a critical section, so that it is executed atomically.