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

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Date:   Tue, 22 May 2018 13:10:09 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     brgerst@...il.com, xen-devel <xen-devel@...ts.xenproject.org>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] xen/PVH: Set up GS segment for stack canary

On 05/22/2018 12:32 PM, Jan Beulich wrote:
>>>> On 22.05.18 at 18:20, <boris.ostrovsky@...cle.com> wrote:
>> On 05/22/2018 12:10 PM, Jan Beulich wrote:
>>>>>> On 22.05.18 at 17:15, <brgerst@...il.com> wrote:
>>>> On Tue, May 22, 2018 at 9:57 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>>>>>> On 22.05.18 at 15:45, <brgerst@...il.com> wrote:
>>>>>> On Mon, May 21, 2018 at 11:54 PM, Boris Ostrovsky <boris.ostrovsky@...cle.com> 
>> 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?
> Oh, that's rather confusing - we're still running on the low 1:1
> mapping when we're here. But yes, by the time we enter C code
> (where the GS base starts to matter) we ought to be on the high
> mappings - if only there wasn't xen_prepare_pvh().

xen_prepare_pvh() (and whatever it might call) is the only reason for
this patch to exist. It's the only C call that we are making before
jumping to startup_64, which I assume will have to set up GS itself
before calling into C.

I didn't realize we are still on identity mapping. I'll clear EDX (and
load $_pa(canary)) then.

BTW, don't we have the same issue in startup_xen()?

-boris