From: Manuel Bouyer <bouyer@antioche.eu.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Jan Beulich" <JBeulich@suse.com>,
"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
"Andy Lutomirski" <luto@kernel.org>
Subject: Re: [PATCH v2] x86/pv: Inject #UD for missing SYSCALL callbacks
Date: Fri, 9 Oct 2020 14:40:57 +0200 [thread overview]
Message-ID: <20201009124057.GC20248@mail.soc.lip6.fr> (raw)
In-Reply-To: <20201009115301.19516-1-andrew.cooper3@citrix.com>
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@antioche.eu.org>
NetBSD: 26 ans d'experience feront toujours la difference
--
next prev parent reply other threads:[~2020-10-09 12:41 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-23 10:18 [PATCH 0/3] x86/pv: Multiple fixes to SYSCALL/SYSENTER handling (XSA-339 followup) Andrew Cooper
2020-09-23 10:18 ` [PATCH 1/3] x86/pv: Don't deliver #GP for a SYSENTER with NT set Andrew Cooper
2020-09-24 13:55 ` Jan Beulich
2020-09-23 10:18 ` [PATCH 2/3] x86/pv: Don't clobber NT on return-to-guest Andrew Cooper
2020-09-24 13:57 ` Jan Beulich
2020-09-23 10:18 ` [PATCH 3/3] x86/pv: Inject #UD for missing SYSCALL callbacks Andrew Cooper
2020-09-24 14:56 ` Jan Beulich
2020-09-28 13:05 ` Andrew Cooper
2020-09-28 15:35 ` Jan Beulich
2020-10-09 11:53 ` [PATCH v2] " Andrew Cooper
2020-10-09 12:40 ` Manuel Bouyer [this message]
2020-10-09 12:50 ` Andrew Cooper
2020-10-14 14:16 ` Roger Pau Monné
2020-10-14 14:20 ` Manuel Bouyer
2020-10-14 14:26 ` Andrew Cooper
2020-10-14 15:17 ` Andrew Cooper
2020-10-14 16:28 ` Roger Pau Monné
2020-10-14 17:41 ` Andrew Cooper
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