linux-kernel - Re: [Xen-devel] [PATCH 3/4] xen/PVH: Set up GS segment for stack canary

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Date:   Wed, 02 May 2018 02:16:13 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:     "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/4] xen/PVH: Set up GS segment for
 stack canary

>>> On 30.04.18 at 18:23, <boris.ostrovsky@...cle.com> wrote:
> --- a/arch/x86/xen/xen-pvh.S
> +++ b/arch/x86/xen/xen-pvh.S
> @@ -54,6 +54,9 @@
>   * charge of setting up it's own stack, GDT and IDT.
>   */
>  
> +#define PVH_GDT_ENTRY_CANARY    4
> +#define PVH_CANARY_SEL          (PVH_GDT_ENTRY_CANARY * 8)

I can only advise against doing it this way: There's no safeguard against
someone changing asm/segment.h without changing this value (in fact
this applies to all of the GDT selectors populated in this file). At the very
least tie this to GDT_ENTRY_BOOT_TSS / __BOOT_TSS?

> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen)
>  	mov %eax,%es
>  	mov %eax,%ss
>  
> +	mov $(PVH_CANARY_SEL),%eax
> +	mov %eax,%gs
> +
>  	/* Stash hvm_start_info. */
>  	mov $_pa(pvh_start_info), %edi
>  	mov %ebx, %esi
> @@ -150,6 +156,7 @@ gdt_start:
>  	.quad 0x00cf9a000000ffff            /* __BOOT_CS */
>  #endif
>  	.quad 0x00cf92000000ffff            /* __BOOT_DS */
> +	.quad 0x0040900000000018            /* PVH_CANARY_SEL */

Without any further code before loading the selector, this points at
physical address 0. Don't you need to add in the base address of
the per-CPU stack_canary?

Jan