Re: [Xen-devel] [PATCH v5 4/9] xen/x86: populate PVHv2 Dom0 physical memory map

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/01/17 14:01, Tim Deegan wrote:
> Hi,
>
> At 13:46 +0000 on 27 Jan (1485524765), Andrew Cooper wrote:
>> The actual behaviour can be determined by putting the TSS on a page
>> boundary, making the previous frame non-readable via EPT, and seeing
>> whether an EPT violation occurs.
> Indeed.  Or likewise with normal pagetables. 
>
>>> Yes, I wonder about the I/O bitmap too.  We don't provide one, or even
>>> enough space for a full one, but the current SDM is pretty clear that
>>> the CPU will try to check it in virtual 8086 mode.
>>>
>>> It may be that all the ports actually used happen to fall in the 128
>>> bytes of zeros that we provide.
>> With an offset of 0, we actually provide 256 bytes of zeros in the
>> bitmap within the TSS limit.
> Sure, or at least 128 bytes of zeros and another 128 bytes of something.

That is a good point.  Nothing prevents a guest exiting vm86 mode, and
using a task switch to move to a new tss, which will cause Xen to write
state back into the vm86_tss, making it no longer a zeroed block of memory.

Despite being owned by the guest, this TSS is actually managed by Xen. 
It should be initialised to defaults each time Xen needs to use it on
behalf of the guest.

>>> Or possibly (both for this and the interrupt bitmap) we are causing
>>> #GP and somehow ending up exiting-and-emulating.  But I don't see
>>> quite what the path is for that.
>> We set IOPL to 3 as well as when entering vm86 to fake up real mode. 
>> This bypasses all I/O bitmap checks (a properly common to ring 3
>> protected tasks as well - See specifically 20.2.7 "Sensitive
>> Instructions"), which means the IN/OUT instructions end up directly at
>> the relevant vmexit case.
> 20.2.8.1 makes it clear that this is not the case -- in virtual 8086
> mode all IN/OUT ops check the bitmap event with IOPL == CPL.

Hmm.  Right you area, which explains why the TSS limit is greater than
0x67. 

If the emulation code were working correctly, the emulator should come
to the same conclusion as hardware and inject a #GP fault.  I suspect it
is more likely that RomBIOS doesn't use a port higher than we have
bitmap space for.

~Andrew

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