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Date:   Wed, 25 Aug 2021 12:07:19 -0500
From:   Michael Roth <michael.roth@amd.com>
To:     Joerg Roedel <jroedel@suse.de>
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Subject: Re: [PATCH Part1 v5 23/38] x86/head/64: set up a startup %gs for
 stack protector
Message-ID: <20210825170719.lygfpbhyvmenohxq@amd.com>
References: <20210820151933.22401-1-brijesh.singh@amd.com>
 <20210820151933.22401-24-brijesh.singh@amd.com>
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On Wed, Aug 25, 2021 at 05:07:26PM +0200, Joerg Roedel wrote:
> On Fri, Aug 20, 2021 at 10:19:18AM -0500, Brijesh Singh wrote:
> >  void __head startup_64_setup_env(unsigned long physbase)
> >  {
> > +	u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
> > +
> 
> This breaks as soon as the compiler decides that startup_64_setup_env()
> needs stack protection too.

Good point.

> 
> And the startup_gs_area is also not needed, there is initial_gs for
> that. 
> 
> What you need is something along these lines (untested):
> 
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index d8b3ebd2bb85..3c7c59bc9903 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -65,6 +65,16 @@ SYM_CODE_START_NOALIGN(startup_64)
>  	leaq	(__end_init_task - FRAME_SIZE)(%rip), %rsp
>  
>  	leaq	_text(%rip), %rdi
> +
> +	movl	$MSR_GS_BASE, %ecx
> +	movq	initial_gs(%rip), %rax
> +	movq	$_text, %rdx
> +	subq	%rdx, %rax
> +	addq	%rdi, %rax
> +	movq	%rax, %rdx
> +	shrq	$32,  %rdx
> +	wrmsr
> +
>  	pushq	%rsi
>  	call	startup_64_setup_env
>  	popq	%rsi
> 
> 
> It loads the initial_gs pointer, applies the fixup on it and loads it
> into MSR_GS_BASE. 

This seems to do the trick, and is probably closer to what the 32-bit
version would look like. Thanks for the suggestion!