[PATCH v2 19/20] x86: efistub: Clear BSS in EFI handover protocol entrypoint

From: Ard Biesheuvel
Date: Mon May 08 2023 - 03:06:52 EST

The so-called EFI handover protocol is value-add from the distros that
permits a loader to simply copy a PE kernel image into memory and call
an alternative entrypoint that is described by an embedded boot_params

Most implementations of this protocol do not bother to check the PE
header for minimum alignment, section placement, etc, and therefore also
don't clear the image's BSS, or even allocate enough memory for it.

Allocating more memory on the fly is rather difficult, but let's at
least clear the BSS explicitly when entering in this manner, so that the
decompressor's pseudo-malloc() does not get confused by global variables
that were not zero-initialized correctly.

Note that we shouldn't clear BSS before calling efi_main() if we are not
booting via the handover protocol, but this entrypoint is no longer
shared with the pure EFI entrypoint so we can ignore that case here.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
arch/x86/boot/compressed/head_32.S | 6 -----
arch/x86/boot/compressed/head_64.S | 2 +-
drivers/firmware/efi/libstub/x86-stub.c | 24 +++++++++++++++++---
3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 3f9b80726070a8e7..cd9587fcd5084f22 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -137,12 +137,6 @@ SYM_FUNC_START(startup_32)
jmp *%eax

- jmp efi_main

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 320e2825ff0b32da..b7599cbbd2ea1136 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -458,7 +458,7 @@ SYM_CODE_END(startup_64)
.org 0x390
and $~0xf, %rsp /* realign the stack */
- call efi_main
+ call efi_handover_entry

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 59076e16c1ac11ee..0528db3e36cf636b 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -891,9 +891,9 @@ static void __noreturn enter_kernel(unsigned long kernel_addr,
* On success, we jump to the relocated kernel directly and never return.
* On failure, we exit to the firmware via efi_exit instead of returning.
-asmlinkage void __noreturn efi_main(efi_handle_t handle,
- efi_system_table_t *sys_table_arg,
- struct boot_params *boot_params)
+static void __noreturn efi_main(efi_handle_t handle,
+ efi_system_table_t *sys_table_arg,
+ struct boot_params *boot_params)
struct setup_header *hdr = &boot_params->hdr;
@@ -1002,3 +1002,21 @@ asmlinkage void __noreturn efi_main(efi_handle_t handle,

efi_exit(handle, status);
+void efi_handover_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
+ struct boot_params *boot_params)
+ extern char _bss[], _ebss[];
+ /* Ensure that BSS is zeroed when booting via the handover protocol */
+ memset(_bss, 0, _ebss - _bss);
+ efi_main(handle, sys_table_arg, boot_params);
+#ifdef CONFIG_X86_32
+extern __alias(efi_handover_entry)
+void efi32_stub_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
+ struct boot_params *boot_params);