[PATCH v8 05/23] x86/decompressor: Avoid magic offsets for EFI handover entrypoint

Ard Biesheuvel
2023-08-02T15:49:42Z

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>
---
arch/x86/boot/compressed/efi_mixed.S | 20 +++++++++++++++++++-
arch/x86/boot/compressed/head_64.S | 18 ------------------
2 files changed, 19 insertions(+), 19 deletions(-)

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index dcc562c8f7f35162..9308b595f6f0a5de 100644
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/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
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index e6880208b9a1b90a..a3f764daf3a3adbc 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/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)

--
2.39.2