CVE-2021-3543: Use After Free

A failing usercopy of the slot uid will lead to a stale entry in the file descriptor table as putunusedfd() won't release it. This enables userland to refer to a dangling 'file' object through that still valid file descriptor, leading to all kinds of use-after-free exploitation scenarios.

Exchanging putunusedfd() for closefd(), ksysclose() or alike won't solve the underlying issue, as the file descriptor might have been replaced in the meantime, e.g. via userland calling close() on it (leading to a NULL pointer dereference in the error handling code as 'fget(enclavefd)' will return a NULL pointer) or by dup2()'ing a completely different file object to that very file descriptor, leading to the same situation: a dangling file descriptor pointing to a freed object -- just in this case to a file object of user's choosing.

Generally speaking, after the call to fdinstall() the file descriptor is live and userland is free to do whatever with it. We cannot rely on it to still refer to our enclave object afterwards. In fact, by abusing userfaultfd() userland can hit the condition without any racing and abuse the error handling in the nitro code as it pleases.