Hi,everyone!Thankyouverymuchforcoming!

My nameisVictorCostan,andIamheretotalkaboutSanctum.

ThisworkwasdoneatMIT’sComputerScienceandAILaboratory,incollaborationwithIliaLebedevandProf.SriniDevadas.

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Sanctum’sgoalistrustedcomputing.Inthisrespect,ourworkbelongstothesamefamilyasTPM,TXT,SGX,andTrustZone.However,Sanctumgivesyouunprecedentedprotectionandunprecedentedcontroloveryourcomputer.

MostofSanctum’ssecuritylogicisinsoftware,notinhardwareormicrocode.Oursoftwarecanbeinspectedbythecomputer’sowner,andisamenabletoformalverification.Mostofthissoftwarecanevenbereplacedbythecomputer’sowner.Thisisanunprecedentedlevelofcontrol.

Sanctum’shardwareextensionscanbeappliedtoanyreasonablymodernRISCprocessorcore.IncombinationwithSanctum’ssoftware,thehardwareextensionsprotectagainstanypracticalsoftwareattacks,includingcachetimingattacks.Thisisanunprecedentedlevelofprotection.

OurprototypetargetstheRISC-Varchitecture,andshowsthatSanctumincursacceptableperformanceoverheads.

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Togetthingsstarted,letmegiveyouaquickoverview oftrustedcomputing,whichistheproblemthatwe’retryingtosolve.

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Sothere’sthisdream,calledsecureremotecomputation.Inthisdream,youcanpackageyourcodeanddataintoabundle,sendthatbundleovertoaremotecomputer,havethecomputerrunyoursoftware,andgettheresultsback.Thethingis,youdon’townthatremotecomputer,soyourelyonmagictoprotectyourcodeanddatafromthecomputer’sowner,andfromtheothersoftwarethatmightberunningonthatcomputer.

Unfortunately,thisisjustadream.Theclosestthingwe’vegotisfullyhomomorphicencryption,andthat’snowherenearlypractical.

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Fortunately,wecanapproximatethatdream,ifwe’rewillingtotrustapieceofhardwareontheremote computer.Thetrusted hardwareestablishesasecurecontainerontheremotecomputer,whichprotectsourcomputationanddatafromuntrustedthirdparties.So,ourcomputationissafe,aslongasweonlysendittotrustedhardware.

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So,wehaveto makesurethatweonlytalktotrustedhardware.We can usesoftwareattestationforthis.

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Veryquickly,softwareattestationmeansthatthereisahardwaremanufacturerthatwetrust,andthemanufactureractsasacertificateauthorityinaPKI.

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Eachpieceoftrustedhardwarehas anattestationkey,andthemanufacturerissuesanendorsementcertificateforthepublicattestationkey.Whenweseethatcertificate,weknowthatwecantrustsignaturesissuedbythedevice’sprivateattestationkey,whichisstoredinsecurehardware.

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Now,whenwewanttosendourcodeovertotheremotecomputer,wewillfirstaskthe trustedhardwaretocreateasecurecontainerforus,andwewillsendachallengemessagetothatsecurecontainer.

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Ourloader codeinthecontainerwillreceivethechallengemessageandcomputearesponsemessage,andthenitwillaskthetrustedhardwaretoproduceanattestationsignature.Theattestationsignaturecoversthemessageproducedbyourcontainer,aswellasameasurementofthesecurecontainer’sinitialstate.Whenwevalidatetheattestationsignature,weareassuredthatwearetalkingtoasecurecontainerthatisinitializedaccordingtoourinstructions,andhostedbyhardwarewetrust.

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WhatI’vesaidso farappliestoalltrustedcomputingsolutions.Thesesolutionsdifferbytheamountofsoftwarethatgoesintoasecurecontainer,bythesecurityguaranteesofferedtothecontainer,bythetrustedhardwareandsoftwareneededtoenforcethesystem’ssecurity,andbythedetailsoftheirsoftwareattestationprocess.

TherestofthistalkwillhopefullyclarifywhereSanctumstandsonalltheseaspects.

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Oursecurecontainersarecalledenclaves,andtheyare conceptuallyextensionsofapplicationprocesses.Enclavesrunatthelowestpossibleprivilegelevel -- thisisknownasring3inx86,orusermodeanywhereelse.Thismeansenclavescannotcompromisethehostcomputer’sOSorhypervisor.Sowedon’tneedtoworryaboutrestrictingenclaves–thesamemechanismsusedtopoliceuserprocesseswillworkforenclaves.

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Enclavescanaccessthememoryspaceoftheirhostapplications,buttheycannotperformsyscalls directly.Thisisbecausewedon’ttrusttheOSkerneltonotdamagetheenclave’sexecutioncontextbeforereturningfromthesyscall.Toworkaroundthis,theenclavemustrelyonitshostapplicationtoproxysyscalls totheOS.Thesyscall proxying requirescodeintheenclave,andinthehostapplication.Weexpectthiscodetobecomeapartoftheruntimelibrary.Forexample,ifyourenclavesoftwareuseslibc,you’dsimplyuseanenclave-awarelibc thatproxiesallthesyscallsitmakes.

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Ourenclavesaren’tnew.SGX’senclavesoperateinaverysimilarway.Somethingthat_is_new,andthatI’mreallyproudof– thevastmajorityoursecuritylogicisexpressedinsoftware.Thisisn’tfirmwareormicrocode.It’snormalsoftwarethatexecutesusingtheprocessor’sstandardexecutionfacilities.It’ssoftwarethatthecomputer’sownercaninspectandanalyze.

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Thedownsideofnotrelyingonfirmwareormicrocodeisthatoursoftwareisnotautomaticallyisolatedfromrestofthecoderunningonthemachine,andwemayberunningalongsideamalicioushypervisororoperatingsystem.Inordertobeabletoprovideanysortofmeaningfulsecurityguarantees,weassumethatoursoftwarerunsatahigherprivilegelevelthananythingelse.TheRISC-Varchitectureconvenientlyincludesamachinelevel,andourprototypetakesadvantageoftheexistenceofthismachinelevel.

Weruntwopiecesofsoftwareatthemachinelevel.

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Thecentralpieceofsoftwareisthesecuritymonitor,whichenforcesSanctum’ssecuritypolicies.Oursecuritymonitoristiny,soitcanbeamenabletoformalverification.Thekeytokeepingitsosmallisthatwedon’tmakeanyresourceallocationdecisionsinthemonitor.Theoperatingsystemgetstomakeallthedecisions,justliketoday.InSanctum,theOSmustsubmititsdecisionstothesecuritymonitorinordertohavethemapplied.Thisway,wecanverifytheOS’decisionsinthemonitor,andwecanrejectdecisionsthatwouldbreakSanctum’ssecurityguarantees.

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Theotherpieceofsoftwarethatrunsinmachinemodeisthemeasurementroot.Thiscoderunsatboottime,andsetsupthesoftwareattestationchain.Itdoesnotplayanypartafterthebootprocesscompletes.

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Let’sseehowSanctum givescomputerownersunprecedentedcontrolovertheirsystems.

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Beforewegointoattestation,let’stalkalittlebitabout Sanctum’sbootprocess.OneofthethingsthatmakeSanctumspecialisthatthesecuritymonitor,whichismostofourlogic,isstoredinflashmemory,andcanbereplacedbythecomputer’sowner.

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Inordertokeepthingssecure,werelyonthemeasurementrootcode,whichisstoredintheprocessor’sROM,andcannotbemodified.

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Whenthecomputerpowersup,themeasurementrootreadsthesecuritymonitorandcomputesitscryptographichash.

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ThishashisfedintosomecryptographicmachinerythateventuallygeneratesanRSAkeypair,whichbecomesthemonitor’sattestationkey.

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Finally,themeasurementrootusestheprocessor’sattestationkeytoissueacertificatethatcontainsthemonitor’spublicattestationkeyanditsmeasurementhash.Afterthishappens,themeasurementroothandscontrolovertothesecuritymonitor. Thekeyfacttonotehereisthatthemonitor’scertificatecontainsthemonitor’smeasurement.Ifthecomputerownermodifiesthemonitor,thechangeswillbereflectedintheattestationcertificate.

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Ifyou’rehorrifiedbytheprospectofhavingtogenerateanRSAkeyoneveryboot,restassured.Wehaveamechanismtosecurelycachethemonitor’sRSAkeyinflash.Thisway,weonlyneedtoregeneratetheRSAkeywhenthecomputerownerreplacesthesecuritymonitor,whichshouldn’thappenveryoften.

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WeborrowedSGX’sgeneralapproachtoenclavemeasurement.Whenanenclaveiscreated,itisinaloadingstate, whereitcannotbeused.TheapplicationthatcreatedtheenclaveworkswiththeOStoallocateresourcestotheenclave,andtoloadtheinitialpagesofcodeanddataintotheenclave.Theparametersofeachloadingoperationarehashedandcontributetotheenclave’smeasurement.

Whentheloadingstageiscomplete,theenclaveisinitialized.Atthispoint,theenclave’smeasurementisfinalized,andthesecuritymonitorwillnotallowtheuseofanyloadingAPIontheenclave.So,anenclave’smeasurementisanaccuraterepresentationoftheenclave’sinitialmemorystate.

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Nowthat youknowhowthebootprocessworks,you’llfindtherestoftheattestationtobeprettystandard.I’llgothroughitquickly,soyoucanseetheideasI’veoutlinedcometogether.

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First,youtrust theSanctumprocessormanufacturer,whichhasarootkey.

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EachSanctumprocessorhasa processorattestationkey,andanendorsementcertificatefromthemanufacturer.

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Whenthecomputer powersup,themeasurementrootcreatesamonitorattestationkeypairandissuesanattestationcertificatetothemonitor.

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Now,whenyouwanttosendyourcomputationover toaSanctumprocessor,yousendtheenclave’sinitialcontents,andachallengemessage.Theenclavestartsexecuting,readsthechallengemessage,andcomputestheresponsemessage.TheenclaveusesSanctum’ssecureinter-enclavecommunicationprimitivestosendahashofthechallengeresponsetotheSigningEnclave,whichisaspecialenclavethatthesecuritymonitortrusts.

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InSanctum,thesecuritymonitordoesnotperformanyoperationsusingtheprivateattestationkey.Instead,thereisaspecialSigningEnclave,whosemeasurementhashishard-codedintothesecuritymonitor.Thisistheonlyenclavethatcanreceivethemonitor’sprivateattestationkey.

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So,ourenclavemustuseSanctum’ssecureinter-enclavecommunicationprimitivestosendahashoftheresponsemessagetotheSigningEnclave.TheSigningEnclavecreatesanattestationsignaturethatcoversourenclave’smeasurementandtheresponsemessage.

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Atthispoint,wecanexaminetheattestationsignature,andthemonitorandprocessor’sattestationcertificates,toconvinceourselvesthatwearecommunicatingwithanenclavebuiltaccordingtoourinstructions,hostedonaSanctumprocessorrunningasecuritymonitorthatwetrust.

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Now,let’stalkabitaboutprotection.

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Sanctumwasdesignedformulti-coreprocessorswithasharedlast-levelcache.Thismattersbecauseper-corecachescanbeprotectedprettyeasily– wheneveryouenterorexitanenclave,flushthecore’scaches.Thisisasimplestrategy,andwelikesimple,sothisisexactlywhatwedotoprotectper-corecaches.

ThesharedLLCismoreinteresting,becauseanattackerthreadcouldbeattemptingatimingattackatanytime.Sanctumusesaverysimplescheme,calledpagecoloring,whichIwilloutlinesoon.

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Sanctum*almost*scalestoafull-fledgedInteldesktopsystem.Theoneaspectwedon’tsupportishyper-threading.Thisisbecausewithhyper-threading,there’sjustsimplytoomuchmicro-architectural resourcesharing.Wecouldn’tcomeupwithanysanewaytoprotectanenclavethreadfrombeingattackedbyathreadrunningonthesamecore.SoSanctumdoesn’tsupporthyper-threading.

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Now,let’sturnontocachetimingattacks.Ataveryhigh level,theseattackstakeadvantageofthefactthatacachehitismuchfasterthanacachemiss.Virtuallyallcachetimingattacksrequiretheattackertoaccessthesamecacheasthevictim,sothememoryaccesstimingcanbeobserved.

Sanctumtargetsset-associativecaches,whicharethemostcommonlyusedcaches.Forthepurposeofthistalk,allthatmattersisthataset-associativecacheismadeupofmultiplesets,andamemorylocationcanonlybecachedinexactlyoneset,dependingonitsaddress.Conceptually,wecanpretendthateachsetinaset-associativecacheisaseparatecachememory.

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This istheinsightbehindcachepartitioning.Ifwecanensurethatmemoryownedbyanenclavewillneverendupinthesamecachesetasmemoryownedbyanattacker,wecanconsiderthattheenclaveandtheattackerareusingdifferentcaches.Thisdefeatscachetimingattacks,becausetheyrequireasharedcache.

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Set-associativecaches usephysicalmemoryaddressesforsetindexcomputation.

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Thisisconvenientforus,becausesoftwareusesaddresstranslation,whichabstractsawayphysicaladdressesalmostcompletely.Wecanasktheoperating systemtostructureitspagetablesinanywaythathelpsusachieveoursecuritygoal.

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PagecoloringessentiallycomesdowntothisobservationthatIjustmade.Therearesomephysical addressbitsthatbelongtothecachesetindex,andaresetbythepagetables.Thesebitscanbeusedtocontrolcacheplacement.

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Theproblemis,bydefault,thepagecolorbitsareatthebottomofanaddress’physicalpagenumber.IfyoudrawyourDRAMandcoloreachpage,it’sgoingtolooklikethestackontheleft.

Thisisaproblembecause,inSanctum,we’dliketoassignsomecolorsexclusivelytoenclaves.Atthesametime,theoperatingsystemprobablyneedssomelargecontinuouschunksofDRAMforDMA.Forexample,ifyouhaveagraphicscardorahigh-performancenetworkcard,thesethingstendtolikelargeDMAbuffers.

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Weuseabitofhardwaretomungethephysicaladdressesastheyenterthecacheunit,andobtainthecolormapontheright.

InSanctum,theDRAMiseffectivelysplitintoequally-sizedDRAMregions,andeachregionmapstodisjointLLCsets.

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EachDRAMregioncanbeassignedtoexactlyoneenclave,ortotheoperatingsystemanditsuntrustedprocesses.Mostofeachenclave’svirtualaddressspaceismappedintothehostapplication’smemory,usingthepagetablespreparedbytheOSforthehostapplication.However,onecontinuousrangeisusedtomaptheenclave’sownDRAMregions,usingaseparatesetofpagetablesthatismanagedbytheenclave.

Thisgivesusisolationatalllevels.Thedatainanenclave’sDRAMregionsisnotaccessibletoanyothersoftware.Thepagetablesusedtomaptheenclave’sprivatememoryarealsoisolated,sotheydon’tleakmemoryaccesspatterns.

…andthiswrapsupmyhigh-leveloverview.Theexcitingdetailsareinthepaper!

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