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

[Boris Ostrovsky]

On 05/02/2018 11:41 AM, Jan Beulich wrote:
>>>> 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. 


Would creating a canary variable and using it as a base address be better?


-boris