selftests/x86: Add shadow stack test

Add a simple selftest for exercising some shadow stack behavior:
 - map_shadow_stack syscall and pivot
 - Faulting in shadow stack memory
 - Handling shadow stack violations
 - GUP of shadow stack memory
 - mprotect() of shadow stack memory
 - Userfaultfd on shadow stack memory
 - 32 bit segmentation
 - Guard gap test
 - Ptrace test

Co-developed-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Rick Edgecombe <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Acked-by: Mike Rapoport (IBM) <[email protected]>
Tested-by: Pengfei Xu <[email protected]>
Tested-by: John Allen <[email protected]>
Tested-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/all/20230613001108.3040476-40-rick.p.edgecombe%40intel.com

Author: rpedgeco

tools/testing/selftests/x86/Makefile

@@ -18,7 +18,7 @@ TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
 TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering \
-			corrupt_xstate_header amx lam
+			corrupt_xstate_header amx lam test_shadow_stack
 # Some selftests require 32bit support enabled also on 64bit systems
 TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall