Commit 15675706 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Borislav Petkov (AMD)
Browse files

x86/startup_64: Drop long return to initial_code pointer 



Since

  866b556e ("x86/head/64: Install startup GDT")

the primary startup sequence sets the code segment register (CS) to
__KERNEL_CS before calling into the startup code shared between primary
and secondary boot.

This means a simple indirect call is sufficient here.

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/20240129180502.4069817-24-ardb+git@google.com
parent 41bccc98

arch/x86/kernel/head_64.S

+3 −32
Original line number Diff line number Diff line
@@ -428,39 +428,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL) 
	movq	%r15, %rdi



.Ljump_to_C_code:

	/*

	 * Jump to run C code and to be on a real kernel address.

	 * Since we are running on identity-mapped space we have to jump

	 * to the full 64bit address, this is only possible as indirect

	 * jump.  In addition we need to ensure %cs is set so we make this

	 * a far return.

	 *

	 * Note: do not change to far jump indirect with 64bit offset.

	 *

	 * AMD does not support far jump indirect with 64bit offset.

	 * AMD64 Architecture Programmer's Manual, Volume 3: states only

	 *	JMP FAR mem16:16 FF /5 Far jump indirect,

	 *		with the target specified by a far pointer in memory.

	 *	JMP FAR mem16:32 FF /5 Far jump indirect,

	 *		with the target specified by a far pointer in memory.

	 *

	 * Intel64 does support 64bit offset.

	 * Software Developer Manual Vol 2: states:

	 *	FF /5 JMP m16:16 Jump far, absolute indirect,

	 *		address given in m16:16

	 *	FF /5 JMP m16:32 Jump far, absolute indirect,

	 *		address given in m16:32.

	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,

	 *		address given in m16:64.

	 */

	pushq	$.Lafter_lret	# put return address on stack for unwinder

	xorl	%ebp, %ebp	# clear frame pointer

	movq	initial_code(%rip), %rax

	pushq	$__KERNEL_CS	# set correct cs

	pushq	%rax		# target address in negative space

	lretq

.Lafter_lret:

	ANNOTATE_NOENDBR

	ANNOTATE_RETPOLINE_SAFE

	callq	*initial_code(%rip)

	ud2

SYM_CODE_END(secondary_startup_64)



#include "verify_cpu.S"