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Re: [Xen-devel] Ongoing/future speculative mitigation work

  • To: Wei Liu <wei.liu2@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Mon, 10 Dec 2018 12:12:34 +0000
  • Autocrypt: addr=george.dunlap@xxxxxxxxxx; prefer-encrypt=mutual; keydata= xsFNBFPqG+MBEACwPYTQpHepyshcufo0dVmqxDo917iWPslB8lauFxVf4WZtGvQSsKStHJSj 92Qkxp4CH2DwudI8qpVbnWCXsZxodDWac9c3PordLwz5/XL41LevEoM3NWRm5TNgJ3ckPA+J K5OfSK04QtmwSHFP3G/SXDJpGs+oDJgASta2AOl9vPV+t3xG6xyfa2NMGn9wmEvvVMD44Z7R W3RhZPn/NEZ5gaJhIUMgTChGwwWDOX0YPY19vcy5fT4bTIxvoZsLOkLSGoZb/jHIzkAAznug Q7PPeZJ1kXpbW9EHHaUHiCD9C87dMyty0N3TmWfp0VvBCaw32yFtM9jUgB7UVneoZUMUKeHA fgIXhJ7I7JFmw3J0PjGLxCLHf2Q5JOD8jeEXpdxugqF7B/fWYYmyIgwKutiGZeoPhl9c/7RE Bf6f9Qv4AtQoJwtLw6+5pDXsTD5q/GwhPjt7ohF7aQZTMMHhZuS52/izKhDzIufl6uiqUBge 0lqG+/ViLKwCkxHDREuSUTtfjRc9/AoAt2V2HOfgKORSCjFC1eI0+8UMxlfdq2z1AAchinU0 eSkRpX2An3CPEjgGFmu2Je4a/R/Kd6nGU8AFaE8ta0oq5BSFDRYdcKchw4TSxetkG6iUtqOO ZFS7VAdF00eqFJNQpi6IUQryhnrOByw+zSobqlOPUO7XC5fjnwARAQABzSRHZW9yZ2UgVy4g RHVubGFwIDxkdW5sYXBnQHVtaWNoLmVkdT7CwYAEEwEKACoCGwMFCwkIBwMFFQoJCAsFFgID AQACHgECF4ACGQEFAlpk2IEFCQo9I54ACgkQpjY8MQWQtG1A1BAAnc0oX3+M/jyv4j/ESJTO U2JhuWUWV6NFuzU10pUmMqpgQtiVEVU2QbCvTcZS1U/S6bqAUoiWQreDMSSgGH3a3BmRNi8n HKtarJqyK81aERM2HrjYkC1ZlRYG+jS8oWzzQrCQiTwn3eFLJrHjqowTbwahoiMw/nJ+OrZO /VXLfNeaxA5GF6emwgbpshwaUtESQ/MC5hFAFmUBZKAxp9CXG2ZhTP6ROV4fwhpnHaz8z+BT NQz8YwA4gkmFJbDUA9I0Cm9D/EZscrCGMeaVvcyldbMhWS+aH8nbqv6brhgbJEQS22eKCZDD J/ng5ea25QnS0fqu3bMrH39tDqeh7rVnt8Yu/YgOwc3XmgzmAhIDyzSinYEWJ1FkOVpIbGl9 uR6seRsfJmUK84KCScjkBhMKTOixWgNEQ/zTcLUsfTh6KQdLTn083Q5aFxWOIal2hiy9UyqR VQydowXy4Xx58rqvZjuYzdGDdAUlZ+D2O3Jp28ez5SikA/ZaaoGI9S1VWvQsQdzNfD2D+xfL qfd9yv7gko9eTJzv5zFr2MedtRb/nCrMTnvLkwNX4abB5+19JGneeRU4jy7yDYAhUXcI/waS /hHioT9MOjMh+DoLCgeZJYaOcgQdORY/IclLiLq4yFnG+4Ocft8igp79dbYYHkAkmC9te/2x Kq9nEd0Hg288EO/OwE0EVFq6vQEIAO2idItaUEplEemV2Q9mBA8YmtgckdLmaE0uzdDWL9To 1PL+qdNe7tBXKOfkKI7v32fe0nB4aecRlQJOZMWQRQ0+KLyXdJyHkq9221sHzcxsdcGs7X3c 17ep9zASq+wIYqAdZvr7pN9a3nVHZ4W7bzezuNDAvn4EpOf/o0RsWNyDlT6KECs1DuzOdRqD oOMJfYmtx9hMzqBoTdr6U20/KgnC/dmWWcJAUZXaAFp+3NYRCkk7k939VaUpoY519CeLrymd Vdke66KCiWBQXMkgtMGvGk5gLQLy4H3KXvpXoDrYKgysy7jeOccxI8owoiOdtbfM8TTDyWPR Ygjzb9LApA8AEQEAAcLBZQQYAQoADwIbDAUCWmTXMwUJB+tP9gAKCRCmNjwxBZC0bb+2D/9h jn1k5WcRHlu19WGuH6q0Kgm1LRT7PnnSz904igHNElMB5a7wRjw5kdNwU3sRm2nnmHeOJH8k Yj2Hn1QgX5SqQsysWTHWOEseGeoXydx9zZZkt3oQJM+9NV1VjK0bOXwqhiQyEUWz5/9l467F S/k4FJ5CHNRumvhLa0l2HEEu5pxq463HQZHDt4YE/9Y74eXOnYCB4nrYxQD/GSXEZvWryEWr eDoaFqzq1TKtzHhFgQG7yFUEepxLRUUtYsEpT6Rks2l4LCqG3hVD0URFIiTyuxJx3VC2Ta4L H3hxQtiaIpuXqq2D4z63h6vCx2wxfZc/WRHGbr4NAlB81l35Q/UHyMocVuYLj0llF0rwU4Aj iKZ5qWNSEdvEpL43fTvZYxQhDCjQTKbb38omu5P4kOf1HT7s+kmQKRtiLBlqHzK17D4K/180 ADw7a3gnmr5RumcZP3NGSSZA6jP5vNqQpNu4gqrPFWNQKQcW8HBiYFgq6SoLQQWbRxJDHvTR YJ2ms7oCe870gh4D1wFFqTLeyXiVqjddENGNaP8ZlCDw6EU82N8Bn5LXKjR1GWo2UK3CjrkH pTt3YYZvrhS2MO2EYEcWjyu6LALF/lS6z6LKeQZ+t9AdQUcILlrx9IxqXv6GvAoBLJY1jjGB q+/kRPrWXpoaQn7FXWGfMqU+NkY9enyrlw==
  • Cc: Martin Pohlack <mpohlack@xxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Joao Martins <joao.m.martins@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Daniel Kiper <daniel.kiper@xxxxxxxxxx>, Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Anthony Liguori <aliguori@xxxxxxxxxx>, "Dannowski, Uwe" <uwed@xxxxxxxxx>, Lars Kurth <lars.kurth@xxxxxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>, Ross Philipson <ross.philipson@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Matt Wilson <msw@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Juergen Gross <JGross@xxxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Xen-devel List <xen-devel@xxxxxxxxxxxxx>, Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>, "Woodhouse, David" <dwmw@xxxxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Mon, 10 Dec 2018 12:13:04 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 12/7/18 6:40 PM, Wei Liu wrote:
> On Thu, Oct 18, 2018 at 06:46:22PM +0100, Andrew Cooper wrote:
>> Hello,
>> This is an accumulation and summary of various tasks which have been
>> discussed since the revelation of the speculative security issues in
>> January, and also an invitation to discuss alternative ideas.  They are
>> x86 specific, but a lot of the principles are architecture-agnostic.
>> 1) A secrets-free hypervisor.
>> Basically every hypercall can be (ab)used by a guest, and used as an
>> arbitrary cache-load gadget.  Logically, this is the first half of a
>> Spectre SP1 gadget, and is usually the first stepping stone to
>> exploiting one of the speculative sidechannels.
>> Short of compiling Xen with LLVM's Speculative Load Hardening (which is
>> still experimental, and comes with a ~30% perf hit in the common case),
>> this is unavoidable.  Furthermore, throwing a few array_index_nospec()
>> into the code isn't a viable solution to the problem.
>> An alternative option is to have less data mapped into Xen's virtual
>> address space - if a piece of memory isn't mapped, it can't be loaded
>> into the cache.
>> An easy first step here is to remove Xen's directmap, which will mean
>> that guests general RAM isn't mapped by default into Xen's address
>> space.  This will come with some performance hit, as the
>> map_domain_page() infrastructure will now have to actually
>> create/destroy mappings, but removing the directmap will cause an
>> improvement for non-speculative security as well (No possibility of
>> ret2dir as an exploit technique).
>> Beyond the directmap, there are plenty of other interesting secrets in
>> the Xen heap and other mappings, such as the stacks of the other pcpus. 
>> Fixing this requires moving Xen to having a non-uniform memory layout,
>> and this is much harder to change.  I already experimented with this as
>> a meltdown mitigation around about a year ago, and posted the resulting
>> series on Jan 4th,
>> https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg00274.html,
>> some trivial bits of which have already found their way upstream.
>> To have a non-uniform memory layout, Xen may not share L4 pagetables. 
>> i.e. Xen must never have two pcpus which reference the same pagetable in
>> %cr3.
>> This property already holds for 32bit PV guests, and all HVM guests, but
>> 64bit PV guests are the sticking point.  Because Linux has a flat memory
>> layout, when a 64bit PV guest schedules two threads from the same
>> process on separate vcpus, those two vcpus have the same virtual %cr3,
>> and currently, Xen programs the same real %cr3 into hardware.
>> If we want Xen to have a non-uniform layout, are two options are:
>> * Fix Linux to have the same non-uniform layout that Xen wants
>> (Backwards compatibility for older 64bit PV guests can be achieved with
>> xen-shim).
>> * Make use XPTI algorithm (specifically, the pagetable sync/copy part)
>> forever more in the future.
>> Option 2 isn't great (especially for perf on fixed hardware), but does
>> keep all the necessary changes in Xen.  Option 1 looks to be the better
>> option longterm.
>> As an interesting point to note.  The 32bit PV ABI prohibits sharing of
>> L3 pagetables, because back in the 32bit hypervisor days, we used to
>> have linear mappings in the Xen virtual range.  This check is stale
>> (from a functionality point of view), but still present in Xen.  A
>> consequence of this is that 32bit PV guests definitely don't share
>> top-level pagetables across vcpus.
> Correction: 32bit PV ABI prohibits sharing of L2 pagetables, but L3
> pagetables can be shared. So guests will schedule the same top-level
> pagetables across vcpus. >
> But, 64bit Xen creates a monitor table for 32bit PAE guest and put the
> CR3 provided by guest to the first slot, so pcpus don't share the same
> L4 pagetables. The property we want still holds.

Ah, right -- but Xen can get away with this because in PAE mode, "L3" is
just 4 entries that are loaded on CR3-switch and not automatically kept
in sync by the hardware; i.e., the OS already needs to do its own
"manual syncing" if it updates any of the L3 entires; so it's the same
for Xen.

>> Juergen/Boris: Do you have any idea if/how easy this infrastructure
>> would be to implement for 64bit PV guests as well?  If a PV guest can
>> advertise via Elfnote that it won't share top-level pagetables, then we
>> can audit this trivially in Xen.
> After reading Linux kernel code, I think it is not going to be trivial.
> As now threads in Linux share one pagetable (as it should be).
> In order to make each thread has its own pagetable while still maintain
> the illusion of one address space, there needs to be synchronisation
> under the hood.
> There is code in Linux to synchronise vmalloc, but that's only for the
> kernel portion. The infrastructure to synchronise userspace portion is
> missing.
> One idea is to follow the same model as vmalloc -- maintain a reference
> pagetable in struct mm and a list of pagetables for threads, then
> synchronise the pagetables in the page fault handler. But this is
> probably a bit hard to sell to Linux maintainers because it will touch a
> lot of the non-Xen code, increase complexity and decrease performance.

Sorry -- what do you mean "synchronize vmalloc"?  If every thread has a
different view of the kernel's vmalloc area, then every thread must have
a different L4 table, right?  And if every thread has a different L4
table, then we've already got the main thing we need from Linux, don't we?


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