Re: Bisected Xen-unstable: "Segment register inaccessible for d1v0" when starting HVM guest on intel

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 02/07/14 10:55, Jan Beulich wrote:
>>>> On 02.07.14 at 11:44, <andrew.cooper3@citrix.com> wrote:
>> On 02/07/14 10:28, Jan Beulich wrote:
>>> This being a PV extension to the base architecture, the hardware
>>> specification is meaningless. What we need to do here is _extend_ what
>>> the hardware has specified for those extra accesses. We have three
>>> options basically:
>>> 1) never do any checking on such accesses
>>> 2) honor CPL and EFLAGS.AC
>>> 3) always do the checking
>>> The first one obviously is bad from a security POV. Since the third one is
>>> more strict than the second and since I assume adding some override is
>>> going to be the simpler change than altering the point in time when the
>>> VMCS gets loaded during context switch (the suggestion of which no one
>>> at all commented on so far), I'd prefer that one, but wouldn't mind
>>> option 2 to be implemented instead.
>> The problem is not the hypervisor check.  We are already deep within an
>> hvm_copy_to_user() which is between a stac()/clac() pair.
>>
>> The issue is that guest_walk_tables() is checking a Xen access using
>> guest page tables as if it were a supervisor access given the current
>> context of the vcpu.
> And I only ever referred to the checking done there; the hypervisor
> access is of no concern here.
>
>> What can/should Xen do if its emulated access fails with a guest SMAP
>> violations?  It certainly can't/shouldn't inject a pagefault, nor should
>> it actually fail the write.  copy_to_user() is not subject to the guest
>> operating mode and whether we are writing into guest user or supervisor
>> pages.
> Just like copy_to_user() would produce -EFAULT for a hypercall
> when used on a non-present page or a non-canonical address, it
> should (and afaict will with how things are right now) similarly
> produce -EFAULT for an attempted access to a guest-accessible
> page when the current mode of the guest is supervisor.
>
> To me it is a logical extension to also fail accesses outside of
> hypercalls or emulation.
>
> Jan

Consider an HVM guest with SMAP in effect, making a hypercall.  If a
guest handle points to guest userspace, Xen would be unable to ever
complete the hypercall without an -EFAULT.

I don't think this is reasonable to fail.

~Andrew