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Re: [Xen-devel] [PATCH 00/14] XSA-277 followup

  • To: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 21 Nov 2018 21:22:25 +0000
  • Autocrypt: addr=andrew.cooper3@xxxxxxxxxx; prefer-encrypt=mutual; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA==
  • Cc: Juergen Gross <JGross@xxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Wed, 21 Nov 2018 21:22:51 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 21/11/2018 17:19, Tamas K Lengyel wrote:
> On Wed, Nov 21, 2018 at 6:21 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> This covers various fixes related to XSA-277 which weren't in security
>> supported areas, and associated cleanup.
>> The biggest issue noticed here is that altp2m's use of hardware #VE support
>> will cause general memory corruption if the guest ever balloons out the 
>> page.  The only safe way I think of doing this is for Xen to alloc annonymous
>> domheap pages for the VEINFO, and for the guest to map them in a similar way
>> to the shared info and grant table frames.
> Since ballooning presents all sorts of problems when used with altp2m
> I would suggest just making the two explicitly incompatible during
> domain creation. Beside the info page being possibly ballooned out the
> other problem is when ballooning causes altp2m views to be reset
> completely, removing mem_access permissions and remapped entries.

If only it were that simple.

For reasons of history and/or poor terminology, "ballooning" means two

1) The act of the Toolstack interacting with the balloon driver inside a
VM, to change the current amount of RAM used by the guest.

2) XENMEM_{increase,decrease}_reservation which are the underlying
hypercalls used by guest kernels.

For the toolstack interaction side of things, this is a mess.  There is
a single xenstore key, and a blind assumption that all guests know what
changes to memory/target mean.  There is no negotiation of whether a
balloon driver is running in the guest, and if one is running, there is
no ability for the balloon driver to nack a request it can't fulfil. 
The sole feedback mechanism which exists is the toolstack looking to see
whether the domain has changed the amount of RAM it is using.

PV guests are fairly "special" by any reasonable judgement.  They are
fully aware of their memory layout , an of changes to it across
migrate.  "Ballooning" was implemented at a time when most computers had
MB of RAM rather than GB, and the knowledge a PV guest had was "I've got
a random set of MFNs which aren't currently used by anything important,
and can be handed back to Xen on request.  Xen guests also have shared
memory constructs such as the shared_info page, and grant tables.  A PV
guest gets access to these by programming the frame straight into to the
pagetables, and Xen's permission model DTRT.

Then HVM guests came along.  For reasons of trying to get things
working, they inherited a lot of same interfaces as PV guests, despite
the fundamental differences in the way they work.  One of the biggest
differences was the fact that HVM guests have their gfn=>mfn space
managed by Xen rather than themselves, and in particular, you can no
longer map shared memory structures in the PV way.

For a shared memory structure to be usable, a mapping has to be put into
the guests P2M, so the guest can create a regular pagetable entry
pointing at it.  For reasons which are beyond me, Xen doesn't have any
knowledge of the guests physical layout, and guests arbitrary mutative
capabilities on their GFN space, but with a hypercall set that has
properties such as a return value of "how many items of this batch
succeeded", and replacement properties rather than error properties when
trying to modify a GFN which already has something in it.

Whatever the reasons, it is commonplace for guests to
decrease_reservation out some RAM to create holes for the shared memory
mappings, because it is the only safe way to avoid irreparably
clobbering something else (especially if you're HVMLoader and in charge
of trying to construct the E820/ACPI tables).

tl;dr If you actually prohibit XENMEM_decrease_reservation, HVM guests
don't boot, and that's long before a balloon driver gets up and running.

Now, all of that said, there are a number of very good reasons why a
host administrator might want to prohibit the guest from having
arbitrary mutative capabilities, chief among them being to prevent the
guest from shattering host superpagpes, but also due to
incompatibilities with some of our more interesting features.

The only way I see of fixing this to teach Xen about the guests gfn
layout (as chosen by the domainbuilder), and include within that "space
which definitely doesn't have anything in, and is safe to put shared
mappings into".  Beyond that, we'll need some administrator level
knowledge of which guests are safe to have XENMEM_decrease_reservation
prohibited, or some interlocks inside Xen to disable unsafe features as
soon as we spot a guest which isn't playing by the new rules.

This probably needs some more more thought, but fundamentally, we have
to undo more than a decades worth of "doing it wrong" which has
percolated through the Xen ecosystem.

I'm half tempted to put together a big hammer bit in the domain creation
path which turns off everything like this (and other areas where we know
Xen is lacking, such as default readability/write-ignore of all MSRs),
after which we'll have a rather a more concrete baseline to discuss what
the guests are actually doing, and how to get them back into a working
state while maintaining architectural.


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