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SubjectRe: [Xen-devel] [PATCH 3/4] xen/PVH: Set up GS segment for stack canary
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On 05/02/2018 11:41 AM, Jan Beulich wrote:
>>>> On 02.05.18 at 17:22, <boris.ostrovsky@oracle.com> wrote:
>> On 05/02/2018 11:01 AM, Jan Beulich wrote:
>>>>>> On 02.05.18 at 17:00, <boris.ostrovsky@oracle.com> wrote:
>>>> On 05/02/2018 04:16 AM, Jan Beulich wrote:
>>>>>>>> On 30.04.18 at 18:23, <boris.ostrovsky@oracle.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.
> 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

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