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 10:10:53 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     <brgerst@...il.com>
Cc:     "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "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 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.

Jan