From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Woodhouse, David" Subject: Re: Ongoing/future speculative mitigation work Date: Mon, 22 Oct 2018 15:09:09 +0000 Message-ID: References: <20181022145534.afuxnfzyovdi3hya@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============2753516746892516327==" Return-path: In-Reply-To: <20181022145534.afuxnfzyovdi3hya@zion.uk.xensource.com> MIME-Version: 1.0 List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" To: "Nuernberger, Stefan" , "andrew.cooper3@citrix.com" , "wei.liu2@citrix.com" Cc: "JGross@suse.com" , "sergey.dyasli@citrix.com" , "lars.kurth@citrix.com" , "sstabellini@kernel.org" , "dfaggioli@suse.com" , "Wilson, Matt" , "konrad.wilk@oracle.com" , "george.dunlap@eu.citrix.com" , "ross.philipson@oracle.com" , "mdontu@bitdefender.com" , "marmarek@invisiblethingslab.com" , "xen-devel@lists.xen.org" , "Pohlack, Martin" , "JBeulich@suse.com" , "Liguori, Anthony" , "joao.m.martins@oracle.com" , "roger.pau@citrix.com" , "julien.grall@arm.com" List-Id: xen-devel@lists.xenproject.org --===============2753516746892516327== Content-Language: en-US Content-Type: multipart/signed; micalg=sha-256; protocol="application/x-pkcs7-signature"; boundary="=-tOKwHZAR+fl/mOHg8w6d" --=-tOKwHZAR+fl/mOHg8w6d Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Adding Stefan to Cc. Should we take this to the spexen or another mailing list? On Mon, 2018-10-22 at 15:55 +0100, Wei Liu wrote: > On Thu, Oct 18, 2018 at 06:46:22PM +0100, Andrew Cooper wrote: > > Hello, > >=20 > > 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. > >=20 > > 1) A secrets-free hypervisor. > >=20 > > 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. > >=20 > > 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. > >=20 > > 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. > >=20 > > 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). >=20 > I have looked into making the "separate xenheap domheap with partial > direct map" mode (see common/page_alloc.c) work but found it not as > straight forward as it should've been. >=20 > Before I spend more time on this, I would like some opinions on if there > is other approach which might be more useful than that mode. >=20 > >=20 > > 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.= =20 > > 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.h= tml, > > some trivial bits of which have already found their way upstream. > >=20 > > To have a non-uniform memory layout, Xen may not share L4 pagetables.= =20 > > i.e. Xen must never have two pcpus which reference the same pagetable i= n > > %cr3. > >=20 > > This property already holds for 32bit PV guests, and all HVM guests, bu= t > > 64bit PV guests are the sticking point. Because Linux has a flat memor= y > > 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. >=20 > Which bit of Linux code are you referring to? If you remember it off the > top of your head, it would save me some time digging around. If not, > never mind, I can look it up myself. >=20 > >=20 > > 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. > >=20 > > 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. >=20 > What is the problem with 1+2 at the same time? I think XPTI can be > enabled / disabled on a per-guest basis? >=20 > Wei. --=-tOKwHZAR+fl/mOHg8w6d Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEE4w ggUuMIIEFqADAgECAhEApEtL31g6H7ROXdOiQnA9RjANBgkqhkiG9w0BAQsFADCBlzELMAkGA1UE BhMCR0IxGzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgG A1UEChMRQ09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhl bnRpY2F0aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0EwHhcNMTcxMjIxMDAwMDAwWhcNMTgxMjIxMjM1 OTU5WjAiMSAwHgYJKoZIhvcNAQkBFhFkd213QGFtYXpvbi5jby51azCCASIwDQYJKoZIhvcNAQEB BQADggEPADCCAQoCggEBAKdGKgXuwKMg2r+i/4BZZC0ddRxNq3xIKTakie/VCSzoO7P17A36ZzUc VMEYPfqDt/65xoc6Tdih+qkY2pNDppZ1DZ8mVrAX6O2O60ZhmXB60wMoDvXPZInvkMOW4drqnje/ 7/NOypn/XQAY+ln4KT+3tHG3TfryyJFMedqC/r29KJlCeeCxIzdtq2j5mN42tvPVv4+p+Kr77uui 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