Commit d7156b98 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Borislav Petkov (AMD)
Browse files

x86/efistub: Clear BSS in EFI handover protocol entrypoint 



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: default avatarArd Biesheuvel <ardb@kernel.org>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-7-ardb@kernel.org
parent 12792064

arch/x86/boot/compressed/efi_mixed.S

+13 −1
Original line number Diff line number Diff line
@@ -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

drivers/firmware/efi/libstub/x86-stub.c

+11 −2
Original line number Diff line number Diff line
@@ -970,12 +970,21 @@ void __noreturn efi_stub_entry(efi_handle_t handle, 
}



#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