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From: Hongyan Xia <hx242@xen.org>
To: Bertrand Marquis <Bertrand.Marquis@arm.com>,
	Julien Grall <julien@xen.org>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>, "paul@xen.org" <paul@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Jan Beulich" <jbeulich@suse.com>,
	xen-devel <xen-devel@lists.xenproject.org>, nd <nd@arm.com>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate
Date: Fri, 29 May 2020 10:49:49 +0100	[thread overview]
Message-ID: <bb5b3e39f59e652dd4040fcfe936334aa1a6d17b.camel@xen.org> (raw)
In-Reply-To: <3B88C76B-6972-4A66-AFDC-0B5C27FBA740@arm.com>

On Fri, 2020-05-29 at 08:13 +0000, Bertrand Marquis wrote:
> Hi Julien,
> 
> > On 28 May 2020, at 19:54, Julien Grall <julien@xen.org> wrote:
> > 
> > Hi Bertrand,
> > 
> > Thank you for the patch.
> > 
> > On 28/05/2020 16:25, Bertrand Marquis wrote:
> > > At the moment on Arm, a Linux guest running with KTPI enabled
> > > will
> > > cause the following error when a context switch happens in user
> > > mode:
> > > (XEN) p2m.c:1890: d1v0: Failed to walk page-table va
> > > 0xffffff837ebe0cd0
> > > This patch is modifying runstate handling to map the area given
> > > by the
> > > guest inside Xen during the hypercall.
> > > This is removing the guest virtual to physical conversion during
> > > context
> > > switches which removes the bug
> > 
> > It would be good to spell out that a virtual address is not stable.
> > So relying on it is wrong.
> > 
> > > and improve performance by preventing to
> > > walk page tables during context switches.
> > 
> > With Secret free hypervisor in mind, I would like to suggest to
> > map/unmap the runstate during context switch.
> > 
> > The cost should be minimal when there is a direct map (i.e on Arm64
> > and x86) and still provide better performance on Arm32.
> 
> Even with a minimal cost this is still adding some non real-time
> behaviour to the context switch.
> But definitely from the security point of view as we have to map a
> page from the guest, we could have accessible in Xen some data that
> should not be there.
> There is a trade here where:
> - xen can protect by map/unmapping
> - a guest which wants to secure his data should either not use it or
> make sure there is nothing else in the page
> 
> That sounds like a thread local storage kind of problematic where we
> want data from xen to be accessible fast from the guest and easy to
> be modified from xen.

Can't we just map it into the per-domain region, so that the mapping
and unmapping of runstate is baked into the per-domain region switch
itself during context switch?

Anyway, I am glad to help with secret-free bits if required, although
my knowledge on the Xen Arm side is limited.

Hongyan



      parent reply	other threads:[~2020-05-29  9:50 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-28 15:25 [RFC PATCH 0/1] Runstate error with KPTI Bertrand Marquis
2020-05-28 15:25 ` [RFC PATCH 1/1] xen: Use a global mapping for runstate Bertrand Marquis
2020-05-28 16:53   ` Roger Pau Monné
2020-05-28 17:19     ` Bertrand Marquis
2020-05-28 19:12       ` Julien Grall
2020-05-29  8:15         ` Bertrand Marquis
2020-05-28 18:54   ` Julien Grall
2020-05-29  7:19     ` Roger Pau Monné
2020-05-29  8:24       ` Bertrand Marquis
2020-05-29  7:35     ` Jan Beulich
2020-05-29  8:32       ` Bertrand Marquis
2020-05-29  8:37         ` Jan Beulich
2020-05-29 13:26         ` Roger Pau Monné
2020-05-29 13:37           ` Julien Grall
2020-05-29 14:36             ` Roger Pau Monné
2020-05-29 10:59       ` Julien Grall
2020-05-29 13:09         ` Roger Pau Monné
2020-05-29  8:13     ` Bertrand Marquis
2020-05-29  8:45       ` Jan Beulich
2020-05-29  9:18         ` Bertrand Marquis
2020-05-29  9:27           ` Roger Pau Monné
2020-05-29 13:53             ` Bertrand Marquis
2020-05-29  9:31           ` Jan Beulich
2020-05-29 10:52           ` Julien Grall
2020-05-29  9:43       ` Julien Grall
2020-05-29 14:02         ` Bertrand Marquis
2020-05-29 14:15           ` Julien Grall
2020-05-29 14:21             ` Bertrand Marquis
2020-05-29  9:49       ` Hongyan Xia [this message]

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