linux-kernel - Re: [PATCH v6 31/76] x86/head/64: Setup MSR_GS

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Message-ID: <20200828181346.GB19342@zn.tnic>
Date:   Fri, 28 Aug 2020 20:13:46 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>,
        Kees Cook <keescook@...omium.org>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v6 31/76] x86/head/64: Setup MSR_GS_BASE before calling
 into C code

On Mon, Aug 24, 2020 at 10:54:26AM +0200, Joerg Roedel wrote:
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 2b2e91627221..800053219054 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -78,6 +78,14 @@ SYM_CODE_START_NOALIGN(startup_64)
>  	call	startup_64_setup_env
>  	popq	%rsi
>  
> +	/*
> +	 * Setup %gs here already to make stack-protector work - it needs to be
> +	 * setup again after the switch to kernel addresses. The address read
> +	 * from initial_gs is a kernel address, so it needs to be adjusted first
> +	 * for the identity mapping.
> +	 */
> +	movl	$MSR_GS_BASE,%ecx

I'm confused: is this missing those three lines:

        movl    initial_gs(%rip),%eax
        movl    initial_gs+4(%rip),%edx
        wrmsr

as it is done in secondary_startup_64 ?

Or why would you otherwise put 0xc0000101 in %ecx and not do anything
with it...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette