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Subject: Re: [PATCH] x86/xen: init %gs very early to avoid page faults with
 stack protector
To:     Juergen Gross <jgross@suse.com>, <linux-kernel@vger.kernel.org>,
        <xen-devel@lists.xenproject.org>
CC:     <boris.ostrovsky@oracle.com>, <stable@vger.kernel.org>
References: <20180201121601.1268-1-jgross@suse.com>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <66ad81d7-0230-d0a5-f4fb-9dbbd8162884@citrix.com>
Date:   Thu, 1 Feb 2018 12:22:05 +0000
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On 01/02/18 12:16, Juergen Gross wrote:
> When running as Xen pv guest %gs is initialized some time after
> C code is started. Depending on stack protector usage this might be
> too late, resulting in page faults.
>
> So setup %gs and MSR_GS_BASE in assembly code already.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/xen/xen-head.S | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 497cc55a0c16..b47d87076efb 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -9,7 +9,9 @@
>  
>  #include <asm/boot.h>
>  #include <asm/asm.h>
> +#include <asm/msr.h>
>  #include <asm/page_types.h>
> +#include <asm/percpu.h>
>  #include <asm/unwind_hints.h>
>  
>  #include <xen/interface/elfnote.h>
> @@ -35,6 +37,18 @@ ENTRY(startup_xen)
>  	mov %_ASM_SI, xen_start_info
>  	mov $init_thread_union+THREAD_SIZE, %_ASM_SP
>  
> +	/* Set up %gs.
> +	 *
> +	 * The base of %gs always points to the bottom of the irqstack
> +	 * union.  If the stack protector canary is enabled, it is
> +	 * located at %gs:40.  Note that, on SMP, the boot cpu uses
> +	 * init data section till per cpu areas are set up.
> +	 */
> +	movl	$MSR_GS_BASE,%ecx
> +	movq	$INIT_PER_CPU_VAR(irq_stack_union),%rax
> +	cdq
> +	wrmsr

You surely want a #ifdef __x86_64__ ?  This path is common to the 32bit
entry as well?

~Andrew

> +
>  	jmp xen_start_kernel
>  END(startup_xen)
>  	__FINIT