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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.07.14 at 11:44, <andrew.cooper3@xxxxxxxxxx> 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


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