selftests/sgx: Enable multiple thread support

selftests/sgx: Enable multiple thread support

Each thread executing in an enclave is associated with a Thread Control
Structure (TCS). The test enclave contains two hardcoded TCS. Each TCS
contains meta-data used by the hardware to save and restore thread specific
information when entering/exiting the enclave.

The two TCS structures within the test enclave share their SSA (State Save
Area) resulting in the threads clobbering each other's data. Fix this by
providing each TCS their own SSA area.

Additionally, there is an 8K stack space and its address is
computed from the enclave entry point which is correctly done for
TCS #1 that starts on the first address inside the enclave but
results in out of bounds memory when entering as TCS #2. Split 8K
stack space into two separate pages with offset symbol between to ensure
the current enclave entry calculation can continue to be used for both
threads.

While using the enclave with multiple threads requires these fixes the
impact is not apparent because every test up to this point enters the
enclave from the first TCS.

More detail about the stack fix:
-------------------------------
Before this change the test enclave (test_encl) looks as follows:

.tcs (2 pages):
(page 1) TCS #1
(page 2) TCS #2

.text (1 page)
One page of code

.data (5 pages)
(page 1) encl_buffer
(page 2) encl_buffer
(page 3) SSA
(page 4 and 5) STACK
encl_stack:

As shown above there is a symbol, encl_stack, that points to the end of the
.data segment (pointing to the end of page 5 in .data) which is also the
end of the enclave.

The enclave entry code computes the stack address by adding encl_stack to
the pointer to the TCS that entered the enclave. When entering at TCS #1
the stack is computed correctly but when entering at TCS #2 the stack
pointer would point to one page beyond the end of the enclave and a #PF
would result when TCS #2 attempts to enter the enclave.

The fix involves moving the encl_stack symbol between the two stack pages.
Doing so enables the stack address computation in the entry code to compute
the correct stack address for each TCS.

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/a49dc0d85401db788a0a3f0d795e848abf3b1f44.1636997631.git.reinette.chatre@intel.com