x86/decompressor: Avoid magic offsets for EFI handover entrypoint

The native 32-bit or 64-bit EFI handover protocol entrypoint offset
relative to the respective startup_32/64 address is described in
boot_params as handover_offset, so that the special Linux/x86 aware EFI
loader can find it there.

When mixed mode is enabled, this single field has to describe this
offset for both the 32-bit and 64-bit entrypoints, so their respective
relative offsets have to be identical. Given that startup_32 and
startup_64 are 0x200 bytes apart, and the EFI handover entrypoint
resides at a fixed offset, the 32-bit and 64-bit versions of those
entrypoints must be exactly 0x200 bytes apart as well.

Currently, hard-coded fixed offsets are used to ensure this, but it is
sufficient to emit the 64-bit entrypoint 0x200 bytes after the 32-bit
one, wherever it happens to reside. This allows this code (which is now
EFI mixed mode specific) to be moved into efi_mixed.S and out of the
startup code in head_64.S.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-6-ardb@kernel.org

Author: ardbiesheuvel

arch/x86/boot/compressed/efi_mixed.S

@@ -140,6 +140,16 @@ SYM_FUNC_START(__efi64_thunk)
 SYM_FUNC_END(__efi64_thunk)
 
 	.code32
+#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
+SYM_FUNC_START(efi32_stub_entry)
+	add	$0x4, %esp		/* Discard return address */
+	popl	%ecx
+	popl	%edx
+	popl	%esi
+	jmp	efi32_entry
+SYM_FUNC_END(efi32_stub_entry)
+#endif
+
 /*
  * EFI service pointer must be in %edi.
  *
@@ -220,7 +230,7 @@ SYM_FUNC_END(efi_enter32)
  * stub may still exit and return to the firmware using the Exit() EFI boot
  * service.]
  */
-SYM_FUNC_START(efi32_entry)
+SYM_FUNC_START_LOCAL(efi32_entry)
 	call	1f
 1:	pop	%ebx
 
@@ -320,6 +330,14 @@ SYM_FUNC_START(efi32_pe_entry)
 	RET
 SYM_FUNC_END(efi32_pe_entry)
 
+#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
+	.org	efi32_stub_entry + 0x200
+	.code64
+SYM_FUNC_START_NOALIGN(efi64_stub_entry)
+	jmp	efi_stub_entry
+SYM_FUNC_END(efi64_stub_entry)
+#endif
+
 	.section ".rodata"
 	/* EFI loaded image protocol GUID */
 	.balign 4

arch/x86/boot/compressed/head_64.S

@@ -294,17 +294,6 @@ SYM_FUNC_START(startup_32)
 	lret
 SYM_FUNC_END(startup_32)
 
-#if IS_ENABLED(CONFIG_EFI_MIXED) && IS_ENABLED(CONFIG_EFI_HANDOVER_PROTOCOL)
-	.org 0x190
-SYM_FUNC_START(efi32_stub_entry)
-	add	$0x4, %esp		/* Discard return address */
-	popl	%ecx
-	popl	%edx
-	popl	%esi
-	jmp	efi32_entry
-SYM_FUNC_END(efi32_stub_entry)
-#endif
-
 	.code64
 	.org 0x200
 SYM_CODE_START(startup_64)
@@ -542,13 +531,6 @@ trampoline_return:
 	jmp	*%rax
 SYM_CODE_END(startup_64)
 
-#if IS_ENABLED(CONFIG_EFI_MIXED) && IS_ENABLED(CONFIG_EFI_HANDOVER_PROTOCOL)
-	.org 0x390
-SYM_FUNC_START(efi64_stub_entry)
-	jmp	efi_stub_entry
-SYM_FUNC_END(efi64_stub_entry)
-#endif
-
 	.text
 SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)