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

From: Andy Lutomirski
Date: Mon Jul 14 2014 - 22:46:46 EST

On Mon, Jul 14, 2014 at 3:23 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 07/14/2014 02:35 PM, Andy Lutomirski wrote:
>> Presumably the problem is here:
>> ENTRY(xen_iret)
>> pushq $0
>> 1: jmp hypercall_iret
>> ENDPATCH(xen_iret)
>> This seems rather unlikely to work on the espfix stack.
>> Maybe espfix64 should be disabled when running on Xen and Xen should
>> implement its own espfix64 in the hypervisor.
> Perhaps the first question is: is espfix even necessary on Xen? How
> does the Xen PV IRET handle returning to a 16-bit stack segment?

Test case here:

It's sigreturn_32 and sigreturn_64. Summary:

(sigreturn_64 always fails unless my SS patch is applied. results
below for sigreturn_64 assume the patch is applied. This is on KVM
(-cpu host) on Sandy Bridge.)

On Xen with espfix, both OOPS intermittently.

On espfix-less kernels (Xen and non-Xen), 16-bit CS w/ 16-bit SS
always fails. Native (32-bit or 64-bit, according to the binary) CS
with 16-bit SS fails for sigreturn_32, but passes for sigreturn_64. I
find this somewhat odd. Native ss always passes.

So I think that Xen makes no difference here, aside from the bug.

That being said, I don't know whether Linux can do espfix64 at all
when Xen is running -- for all I know, the IRET hypercall switches
stacks to a Xen stack.

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