linux-kernel - Re: [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary

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Date:   Thu, 17 May 2018 09:02:44 -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>
Subject: Re: [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary

>>> On 17.05.18 at 16:47, <boris.ostrovsky@...cle.com> wrote:
> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen)
>  	mov %eax,%es
>  	mov %eax,%ss
>  
> +	mov $PVH_CANARY_SEL,%eax
> +	mov %eax,%gs

I doubt this is needed for 64-bit (you could equally well load zero or leave
in place what's there in that case), and loading the selector before setting
the base address in the descriptor won't have the intended effect.

> @@ -150,9 +170,12 @@ gdt_start:
>  	.quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */
>  #endif
>  	.quad GDT_ENTRY(0xc092, 0, 0xfffff) /* __KERNEL_DS */
> +	.quad GDT_ENTRY(0x4090, 0, 0x18)    /* PVH_CANARY_SEL */
>  gdt_end:
>  
> -	.balign 4
> +	.balign 16
> +canary:
> +	.fill 24, 1, 0

This is too little space for 64-bit afaict (the canary lives at offset 40 there
if I can trust asm/processor.h).

Jan