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x86/efistub: Clear BSS in EFI handover protocol entrypoint

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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
structure.

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 at least
clear the BSS region explicitly when entering in this manner, so that
the EFI stub code does not get confused by global variables that were
not zero-initialized correctly.

When booting in mixed mode, this BSS clearing must occur before any
global state is created, so clear it in the 32-bit asm entry point.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-7-ardb@kernel.org
This commit is contained in:
Ard Biesheuvel 2023-08-07 18:27:03 +02:00 committed by Borislav Petkov (AMD)
parent 1279206458
commit d7156b986d
2 changed files with 24 additions and 3 deletions
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14
arch/x86/boot/compressed/efi_mixed.S
View File

@ -142,6 +142,18 @@ SYM_FUNC_END(__efi64_thunk)
.code32
#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
SYM_FUNC_START(efi32_stub_entry)
call 1f
1: popl %ecx
/* Clear BSS */
xorl %eax, %eax
leal (_bss - 1b)(%ecx), %edi
leal (_ebss - 1b)(%ecx), %ecx
subl %edi, %ecx
shrl $2, %ecx
cld
rep stosl
add $0x4, %esp /* Discard return address */
popl %ecx
popl %edx
@ -334,7 +346,7 @@ SYM_FUNC_END(efi32_pe_entry)
.org efi32_stub_entry + 0x200
.code64
SYM_FUNC_START_NOALIGN(efi64_stub_entry)
jmp efi_stub_entry
jmp efi_handover_entry
SYM_FUNC_END(efi64_stub_entry)
#endif

13
drivers/firmware/efi/libstub/x86-stub.c
View File

@ -970,12 +970,21 @@ fail:
}
#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
void efi_handover_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
struct boot_params *boot_params)
{
extern char _bss[], _ebss[];
memset(_bss, 0, _ebss - _bss);
efi_stub_entry(handle, sys_table_arg, boot_params);
}
#ifndef CONFIG_EFI_MIXED
extern __alias(efi_stub_entry)
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);
extern __alias(efi_stub_entry)
extern __alias(efi_handover_entry)
void efi64_stub_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg,
struct boot_params *boot_params);
#endif