linux-kernel - Re: Is espfix64's double-fault thing OK on Xen?

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Message-ID: <CALCETrVz7a7RVLhYTrGvHkJR2dGHJbRuATOaWaXgkrSABR7YFA@mail.gmail.com>
Date:	Mon, 14 Jul 2014 10:15:57 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Is espfix64's double-fault thing OK on Xen?

On Mon, Jul 14, 2014 at 10:11 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Jul 14, 2014 at 10:04 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 07/09/2014 04:17 PM, Andy Lutomirski wrote:
>>> This part in __do_double_fault looks fishy:
>>>
>>>     cmpl $__KERNEL_CS,CS(%rdi)
>>>     jne do_double_fault
>>>
>>> Shouldn't that be:
>>>
>>>     test $3,CS(%rdi)
>>>     jnz do_double_fault
>>>
>>
>> No, it should be fine.  The *only* case where we need to do the espfix
>> magic is when we are on __KERNEL_CS.
>>
>
> IIRC Xen has a somewhat different GDT, and at least the userspace CS
> in IA32_STAR disagrees with normal Linux.  If the kernel CS is also
> strange, then there will be an extra possible CS value here.

There's FLAT_KERNEL_CS64, which is not equal to __KERNEL_CS.  If the
espfix mechanism gets invoked with that CS, then I expect that
something unexpected will happen.

That being said, FLAT_KERNEL_CS64 is CPL3, so my code might not be any better.

--Andy
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