Re: [PATCH v4 1/2] xen/PVH: Set up GS segment for stack canary

[Boris Ostrovsky]

On 05/22/2018 12:10 PM, Jan Beulich wrote:
>>>> On 22.05.18 at 17:15, <brgerst@xxxxxxxxx> wrote:
>> On Tue, May 22, 2018 at 9:57 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 22.05.18 at 15:45, <brgerst@xxxxxxxxx> wrote:
>>>> On Mon, May 21, 2018 at 11:54 PM, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> @@ -98,6 +101,12 @@ ENTRY(pvh_start_xen)
>>>>>         /* 64-bit entry point. */
>>>>>         .code64
>>>>>  1:
>>>>> +       /* Set base address in stack canary descriptor. */
>>>>> +       mov $MSR_GS_BASE,%ecx
>>>>> +       mov $canary, %rax
>>>>> +       cdq
>>>>> +       wrmsr
>>>> CDQ only sign-extends EAX to RAX.  What you really want is to move the
>>>> high 32-bits to EDX (or zero EDX if we can guarantee it is loaded
>>>> below 4G).
>>> What you describe is CDQE (AT&T name: CLTD); CDQ (AT&T: CLTQ)
>>> sign-extends EAX to EDX:EAX.
>> But that would still be wrong, as it would set EDX to 0xFFFFFFFF if
>> the kernel was loaded between 2G and 4G.  Looking closer at the code,
>> we just left 32-bit mode, so we must have been loaded below 4G,
>> therefore EDX must be zero.
> Ah, yes, indeed.


We are loading virtual address for $canary so we will always have EDX
set to 0xffffffff. Isn't that what we want?

-borsi