lkdtm/bugs: Make double-fault test always available

Adjust the DOUBLE_FAULT test to always be available (so test harnesses
don't have to make exceptions more missing tests), and for the
arch-specific tests to "XFAIL" so that test harnesses can reason about
expected vs unexpected failures.

Fixes: b09511c2 ("lkdtm: Add a DOUBLE_FAULT crash type on x86")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202001021226.751D3F869D@keescookSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/bugs.c

@@ -338,13 +338,13 @@ void lkdtm_UNSET_SMEP(void)
 		native_write_cr4(cr4);
 	}
 #else
-	pr_err("FAIL: this test is x86_64-only\n");
+	pr_err("XFAIL: this test is x86_64-only\n");
 #endif
 }
 
-#ifdef CONFIG_X86_32
 void lkdtm_DOUBLE_FAULT(void)
 {
+#ifdef CONFIG_X86_32
 	/*
 	 * Trigger #DF by setting the stack limit to zero.  This clobbers
 	 * a GDT TLS slot, which is okay because the current task will die
@@ -373,6 +373,8 @@ void lkdtm_DOUBLE_FAULT(void)
 	asm volatile ("movw %0, %%ss; addl $0, (%%esp)" ::
 		      "r" ((unsigned short)(GDT_ENTRY_TLS_MIN << 3)));
 
-	panic("tried to double fault but didn't die\n");
-}
+	pr_err("FAIL: tried to double fault but didn't die\n");
+#else
+	pr_err("XFAIL: this test is ia32-only\n");
 #endif
+}