selftests/sgx: Add page permission and exception test

selftests/sgx: Add page permission and exception test

The Enclave Page Cache Map (EPCM) is a secure structure used by the
processor to track the contents of the enclave page cache. The EPCM
contains permissions with which enclave pages can be accessed. SGX
support allows EPCM and PTE page permissions to differ - as long as
the PTE permissions do not exceed the EPCM permissions.

Add a test that:
(1) Creates an SGX enclave page with writable EPCM permission.
(2) Changes the PTE permission on the page to read-only. This should
    be permitted because the permission does not exceed the EPCM
    permission.
(3) Attempts a write to the page. This should generate a page fault
    (#PF) because of the read-only PTE even though the EPCM
    permissions allow the page to be written to.

This introduces the first test of SGX exception handling. In this test
the issue that caused the exception (PTE page permissions) can be fixed
from outside the enclave and after doing so it is possible to re-enter
enclave at original entrypoint with ERESUME.

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>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/3bcc73a4b9fe8780bdb40571805e7ced59e01df7.1636997631.git.reinette.chatre@intel.com