Re: [PATCH v3 03/19] x86/startup_64: Drop long return to initial_code pointer

[Borislav Petkov]

On Mon, Jan 29, 2024 at 07:05:06PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> Since commit 866b556efa12 ("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: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/kernel/head_64.S | 35 ++------------------
>  1 file changed, 3 insertions(+), 32 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index d4918d03efb4..4017a49d7b76 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -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)
> +	int3
>  SYM_CODE_END(secondary_startup_64)
>  
>  #include "verify_cpu.S"

objtool doesn't like it yet:

vmlinux.o: warning: objtool: verify_cpu+0x0: stack state mismatch: cfa1=4+8 cfa2=-1+0

Once we've solved this, I'll take this one even now - very nice cleanup!

Thx.

-- 
Regards/Gruss,
    Boris.

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