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

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Date:   Wed, 2 May 2018 11:22:54 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jan Beulich <JBeulich@...e.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 05/02/2018 11:01 AM, Jan Beulich wrote:
>>>> On 02.05.18 at 17:00, <boris.ostrovsky@...cle.com> wrote:
>> On 05/02/2018 04:16 AM, Jan Beulich wrote:
>>>>>> 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?
>> This GDT is gone soon after we jump into generic x86 startup code.That
>> code will load its own GDT (and then set up per-cpu segments and all that).
> All understood, but why would you set up the per-CPU segment here if
> what you load into the segment register is not usable for the intended
> purpose (until that other code sets up things and reloads the segment
> registers)?

The intended purpose here is to allow stack protector access not to
fail. At this point it doesn't really matter that GS is later used for
per-cpu segment, this code (and this GDT) will not be used when other
CPUs come up.

-boris