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

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Message-Id: <5B0421E502000078001C4B91@prv1-mh.provo.novell.com>
Date:   Tue, 22 May 2018 07:57:57 -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 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.

Jan