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

From: Andy Lutomirski
Date: Mon Jul 14 2014 - 13:11:42 EST

On Mon, Jul 14, 2014 at 10:04 AM, H. Peter Anvin <hpa@xxxxxxxxx> 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.

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