x86, efi: Add dedicated EFI stub entry point

The method used to work out whether we were booted by EFI firmware or
via a boot loader is broken. Because efi_main() is always executed
when booting from a boot loader we will dereference invalid pointers
either on the stack (CONFIG_X86_32) or contained in %rdx
(CONFIG_X86_64) when searching for an EFI System Table signature.

Instead of dereferencing these invalid system table pointers, add a
new entry point that is only used when booting from EFI firmware, when
we know the pointer arguments will be valid. With this change legacy
boot loaders will no longer execute efi_main(), but will instead skip
EFI stub initialisation completely.

[ hpa: Marking this for urgent/stable since it is a regression when
  the option is enabled; without the option the patch has no effect ]

Signed-off-by: Matt Fleming <matt.hfleming@intel.com>
Link: http://lkml.kernel.org/r/1334584744.26997.14.camel@mfleming-mobl1.ger.corp.intel.com
Reported-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org> v3.3

arch/x86/boot/compressed/head_64.S

@@ -200,18 +200,28 @@ ENTRY(startup_64)
 	 * entire text+data+bss and hopefully all of memory.
 	 */
 #ifdef CONFIG_EFI_STUB
-	pushq	%rsi
+	/*
+	 * The entry point for the PE/COFF executable is 0x210, so only
+	 * legacy boot loaders will execute this jmp.
+	 */
+	jmp	preferred_addr
+
+	.org 0x210
 	mov	%rcx, %rdi
 	mov	%rdx, %rsi
 	call	efi_main
-	popq	%rsi
-	cmpq	$0,%rax
-	je	preferred_addr
 	movq	%rax,%rsi
-	call	1f
+	cmpq	$0,%rax
+	jne	2f
 1:
+	/* EFI init failed, so hang. */
+	hlt
+	jmp	1b
+2:
+	call	3f
+3:
 	popq	%rax
-	subq	$1b, %rax
+	subq	$3b, %rax
 	subq	BP_pref_address(%rsi), %rax
 	add	BP_code32_start(%esi), %eax
 	leaq	preferred_addr(%rax), %rax