Re: [PATCH v2] x86/pv: Inject #UD for missing SYSCALL callbacks

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Fri, Oct 09, 2020 at 12:53:01PM +0100, Andrew Cooper wrote:
> Despite appearing to be a deliberate design choice of early PV64, the
> resulting behaviour for unregistered SYSCALL callbacks creates an untenable
> testability problem for Xen.  Furthermore, the behaviour is undocumented,
> bizarre, and inconsistent with related behaviour in Xen, and very liable
> introduce a security vulnerability into a PV guest if the author hasn't
> studied Xen's assembly code in detail.
> 
> There are two different bugs here.
> 
> 1) The current logic confuses the registered entrypoints, and may deliver a
>    SYSCALL from 32bit userspace to the 64bit entry, when only a 64bit
>    entrypoint is registered.
> 
>    This has been the case ever since 2007 (c/s cd75d47348b) but up until
>    2018 (c/s dba899de14) the wrong selectors would be handed to the guest for
>    a 32bit SYSCALL entry, making it appear as if it a 64bit entry all along.
> 
>    Xen would malfunction under these circumstances, if it were a PV guest.
>    Linux would as well, but PVOps has always registered both entrypoints and
>    discarded the Xen-provided selectors.  NetBSD really does malfunction as a
>    consequence (benignly now, but a VM DoS before the 2018 Xen selector fix).

What do you mean with «malfunction» ? A 64bits guest can run 32bit code
just fine, this is part of our daily regression tests.

-- 
Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
     NetBSD: 26 ans d'experience feront toujours la difference
--