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

[Jan Beulich]

>>> On 02.05.18 at 17:22, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/02/2018 11:01 AM, Jan Beulich wrote:
>>>>> On 02.05.18 at 17:00, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 05/02/2018 04:16 AM, Jan Beulich wrote:
>>>>>>> On 30.04.18 at 18:23, <boris.ostrovsky@xxxxxxxxxx> 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.

But the place the canary would live this way is completely wrong. Anyway,
you're the maintainer of this code, so I guess I better shut up now.

Jan