x86/sgx: Tighten accessible memory range after enclave initialization

Before an enclave is initialized the enclave's memory range is unknown.
The enclave's memory range is learned at the time it is created via the
SGX_IOC_ENCLAVE_CREATE ioctl() where the provided memory range is
obtained from an earlier mmap() of /dev/sgx_enclave. After an enclave
is initialized its memory can be mapped into user space (mmap()) from
where it can be entered at its defined entry points.

With the enclave's memory range known after it is initialized there is
no reason why it should be possible to map memory outside this range.

Lock down access to the initialized enclave's memory range by denying
any attempt to map memory outside its memory range.

Locking down the memory range also makes adding pages to an initialized
enclave more efficient. Pages are added to an initialized enclave by
accessing memory that belongs to the enclave's memory range but not yet
backed by an enclave page. If it is possible for user space to map
memory that does not form part of the enclave then an access to this
memory would eventually fail. Failures range from a prompt general
protection fault if the access was an ENCLU[EACCEPT] from within the
enclave, or a page fault via the vDSO if it was another access from
within the enclave, or a SIGBUS (also resulting from a page fault) if
the access was from outside the enclave.

Disallowing invalid memory to be mapped in the first place avoids
preventable failures.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/6391460d75ae79cea2e81eef0f6ffc03c6e9cfe7.1652137848.git.reinette.chatre@intel.com

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