Re: [Xen-devel] [PATCH] x86/xen: init %gs very early to avoid page faults with stack protector

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  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


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