Commit d2d7a54f authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Borislav Petkov (AMD)
Browse files

x86/efistub: Branch straight to kernel entry point from C code 



Instead of returning to the calling code in assembler that does nothing
more than perform an indirect call with the boot_params pointer in
register ESI/RSI, perform the jump directly from the EFI stub C code.
This will allow the asm entrypoint code to be dropped entirely in
subsequent patches.

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/20230807162720.545787-4-ardb@kernel.org
parent 2f69a81a

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

+17 −5
Original line number Diff line number Diff line
@@ -290,7 +290,7 @@ adjust_memory_range_protection(unsigned long start, unsigned long size) 
#define TRAMPOLINE_PLACEMENT_BASE ((128 - 8)*1024)

#define TRAMPOLINE_PLACEMENT_SIZE (640*1024 - (128 - 8)*1024)



void startup_32(struct boot_params *boot_params);

extern const u8 startup_32[], startup_64[];



static void

setup_memory_protection(unsigned long image_base, unsigned long image_size)
@@ -803,10 +803,19 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle) 
	return EFI_SUCCESS;

}



static void __noreturn enter_kernel(unsigned long kernel_addr,

				    struct boot_params *boot_params)

{

	/* enter decompressed kernel with boot_params pointer in RSI/ESI */

	asm("jmp *%0"::"r"(kernel_addr), "S"(boot_params));



	unreachable();

}



/*

 * On success, we return the address of startup_32, which has potentially been

 * relocated by efi_relocate_kernel.

 * On failure, we exit to the firmware via efi_exit instead of returning.

 * On success, this routine will jump to the relocated image directly and never

 * return.  On failure, it will exit to the firmware via efi_exit() instead of

 * returning.

 */

asmlinkage unsigned long efi_main(efi_handle_t handle,

				  efi_system_table_t *sys_table_arg,
@@ -950,7 +959,10 @@ asmlinkage unsigned long efi_main(efi_handle_t handle, 
		goto fail;

	}



	return bzimage_addr;

	if (IS_ENABLED(CONFIG_X86_64))

		bzimage_addr += startup_64 - startup_32;



	enter_kernel(bzimage_addr, boot_params);

fail:

	efi_err("efi_main() failed!\n");