{"api_version":"1","generated_at":"2026-11-04T09:05:09+00:00","cve":"CVE-2026-99120","urls":{"html":"https://cve.report/CVE-2026-99120","api":"https://cve.report/api/cve/CVE-2026-99120.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2026-99120","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-99120"},"summary":{"title":"KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes\n\nWhen running Hyper-V from Windows 11, KVM is seeing the SS bit set\nin nested page fault error codes produced for shadowed NPT.  The AMD\nmanual is unclear about when SS is set, including whether it can be set\nwhen SupervisorShadowStackEn (bit 6) is not set in the MISC_CTL field\nof the VMCB.\n\nNevertheless, the unexpected SS bit causes out of bounds accesses in\npermission_fault(), seen as either WARNs or UBSAN reports:\n\n  kernel: UBSAN: array-index-out-of-bounds in arch/x86/kvm/mmu.h:225:27\n  kernel: index 34 is out of range for type 'u16 [16]'\n\n(in this case, the page fault error code is 34*2=0x44, i.e. SS|U; the\nreport shows SS|U|W happening as well).  Fix the handling of unknown\nerror codes by dropping SS and (with a WARN) any bits not among those\nthat permission_fault() expects to receive.\n\nAt the same time, given the uncertainty about when hardware sets SS,\nfollow the processor's behavior when constructing the fault's error\ncode, and preserve the SS bit as passed to FNAME(walk_addr_generic).\nWhile the behavior is surprising, there's a possibility that Hyper-V\nexpects it (Hyper-V does not even enable CET unless it finds MBEC/GMET!),\nso retain the information when reflecting the fault to L1 instead of\nunconditionally discarding it.\n\nBisection shows that this started happening with commit 687ee95c1b6d (\"KVM:\nnSVM: enable GMET for guests\"), even though SHSTK was available to guests\nsince Linux 6.18.  However, this is not because bit 1 of MISC_CTL (GMET)\nsomehow enables reporting the SS bit; rather, it's simply that Hyper-V\nnever uses CET unless GMET is also present.  The WARN is reproducible\neven with gmet=0, with some hacking of kvm-unit-tests's cet.c testcase\nto ensure that the first access to the shadow stack triggers a page fault.","state":"PUBLISHED","assigner":"Linux","published_at":"2026-10-28 11:17:31","updated_at":"2026-10-28 11:17:31"},"problem_types":[],"metrics":[],"references":[{"url":"https://git.kernel.org/stable/c/e90ab3669181c6469e3647ef2b30ae408275f496","name":"https://git.kernel.org/stable/c/e90ab3669181c6469e3647ef2b30ae408275f496","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://git.kernel.org/stable/c/0caeefed99b37f03251a00eb1fdb403a1cf2cb68","name":"https://git.kernel.org/stable/c/0caeefed99b37f03251a00eb1fdb403a1cf2cb68","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://git.kernel.org/stable/c/d6e08ec2fff45e0ec69f11010851650d7c6282b9","name":"https://git.kernel.org/stable/c/d6e08ec2fff45e0ec69f11010851650d7c6282b9","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-99120","name":"CVE Program record","refsource":"CVE.ORG","tags":["canonical"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99120","name":"NVD vulnerability detail","refsource":"NVD","tags":["canonical","analysis"]}],"affected":[{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 8db428fd5229ba509d515af10d7f33d7d4a54bfe 0caeefed99b37f03251a00eb1fdb403a1cf2cb68 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 8db428fd5229ba509d515af10d7f33d7d4a54bfe d6e08ec2fff45e0ec69f11010851650d7c6282b9 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 8db428fd5229ba509d515af10d7f33d7d4a54bfe e90ab3669181c6469e3647ef2b30ae408275f496 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 6.18","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 6.18 semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 6.18.59 6.18.* semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 7.2.13 7.2.* semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 7.3 * original_commit_for_fix","platforms":[]}],"timeline":[],"solutions":[],"workarounds":[],"exploits":[],"credits":[],"nvd_cpes":[],"vendor_comments":[],"enrichments":{"kev":null,"epss":null,"legacy_qids":[]},"source_records":{"cve_program":{"containers":{"cna":{"affected":[{"defaultStatus":"unaffected","product":"Linux","programFiles":["arch/x86/kvm/mmu.h","arch/x86/kvm/mmu/paging_tmpl.h"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","vendor":"Linux","versions":[{"lessThan":"0caeefed99b37f03251a00eb1fdb403a1cf2cb68","status":"affected","version":"8db428fd5229ba509d515af10d7f33d7d4a54bfe","versionType":"git"},{"lessThan":"d6e08ec2fff45e0ec69f11010851650d7c6282b9","status":"affected","version":"8db428fd5229ba509d515af10d7f33d7d4a54bfe","versionType":"git"},{"lessThan":"e90ab3669181c6469e3647ef2b30ae408275f496","status":"affected","version":"8db428fd5229ba509d515af10d7f33d7d4a54bfe","versionType":"git"}]},{"defaultStatus":"affected","product":"Linux","programFiles":["arch/x86/kvm/mmu.h","arch/x86/kvm/mmu/paging_tmpl.h"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","vendor":"Linux","versions":[{"status":"affected","version":"6.18"},{"lessThan":"6.18","status":"unaffected","version":"0","versionType":"semver"},{"lessThanOrEqual":"6.18.*","status":"unaffected","version":"6.18.59","versionType":"semver"},{"lessThanOrEqual":"7.2.*","status":"unaffected","version":"7.2.13","versionType":"semver"},{"lessThanOrEqual":"*","status":"unaffected","version":"7.3","versionType":"original_commit_for_fix"}]}],"cpeApplicability":[{"nodes":[{"cpeMatch":[{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"6.18.59","versionStartIncluding":"6.18","vulnerable":true},{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"7.2.13","versionStartIncluding":"6.18","vulnerable":true},{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"7.3","versionStartIncluding":"6.18","vulnerable":true}],"negate":false,"operator":"OR"}]}],"descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes\n\nWhen running Hyper-V from Windows 11, KVM is seeing the SS bit set\nin nested page fault error codes produced for shadowed NPT.  The AMD\nmanual is unclear about when SS is set, including whether it can be set\nwhen SupervisorShadowStackEn (bit 6) is not set in the MISC_CTL field\nof the VMCB.\n\nNevertheless, the unexpected SS bit causes out of bounds accesses in\npermission_fault(), seen as either WARNs or UBSAN reports:\n\n  kernel: UBSAN: array-index-out-of-bounds in arch/x86/kvm/mmu.h:225:27\n  kernel: index 34 is out of range for type 'u16 [16]'\n\n(in this case, the page fault error code is 34*2=0x44, i.e. SS|U; the\nreport shows SS|U|W happening as well).  Fix the handling of unknown\nerror codes by dropping SS and (with a WARN) any bits not among those\nthat permission_fault() expects to receive.\n\nAt the same time, given the uncertainty about when hardware sets SS,\nfollow the processor's behavior when constructing the fault's error\ncode, and preserve the SS bit as passed to FNAME(walk_addr_generic).\nWhile the behavior is surprising, there's a possibility that Hyper-V\nexpects it (Hyper-V does not even enable CET unless it finds MBEC/GMET!),\nso retain the information when reflecting the fault to L1 instead of\nunconditionally discarding it.\n\nBisection shows that this started happening with commit 687ee95c1b6d (\"KVM:\nnSVM: enable GMET for guests\"), even though SHSTK was available to guests\nsince Linux 6.18.  However, this is not because bit 1 of MISC_CTL (GMET)\nsomehow enables reporting the SS bit; rather, it's simply that Hyper-V\nnever uses CET unless GMET is also present.  The WARN is reproducible\neven with gmet=0, with some hacking of kvm-unit-tests's cet.c testcase\nto ensure that the first access to the shadow stack triggers a page fault."}],"providerMetadata":{"dateUpdated":"2026-10-28T10:34:36.224Z","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux"},"references":[{"url":"https://git.kernel.org/stable/c/0caeefed99b37f03251a00eb1fdb403a1cf2cb68"},{"url":"https://git.kernel.org/stable/c/d6e08ec2fff45e0ec69f11010851650d7c6282b9"},{"url":"https://git.kernel.org/stable/c/e90ab3669181c6469e3647ef2b30ae408275f496"}],"title":"KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes","x_generator":{"engine":"bippy-1.2.0"}}},"cveMetadata":{"assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","assignerShortName":"Linux","cveId":"CVE-2026-99120","datePublished":"2026-10-28T10:34:36.224Z","dateReserved":"2026-09-25T10:25:14.393Z","dateUpdated":"2026-10-28T10:34:36.224Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"},"nvd":{"publishedDate":"2026-10-28 11:17:31","lastModifiedDate":"2026-10-28 11:17:31","problem_types":[],"metrics":[],"configurations":[]},"legacy_mitre":{"record":{"CveYear":"2026","CveId":"99120","Ordinal":"1","Title":"KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes","CVE":"CVE-2026-99120","Year":"2026"},"notes":[{"CveYear":"2026","CveId":"99120","Ordinal":"1","NoteData":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes\n\nWhen running Hyper-V from Windows 11, KVM is seeing the SS bit set\nin nested page fault error codes produced for shadowed NPT.  The AMD\nmanual is unclear about when SS is set, including whether it can be set\nwhen SupervisorShadowStackEn (bit 6) is not set in the MISC_CTL field\nof the VMCB.\n\nNevertheless, the unexpected SS bit causes out of bounds accesses in\npermission_fault(), seen as either WARNs or UBSAN reports:\n\n  kernel: UBSAN: array-index-out-of-bounds in arch/x86/kvm/mmu.h:225:27\n  kernel: index 34 is out of range for type 'u16 [16]'\n\n(in this case, the page fault error code is 34*2=0x44, i.e. SS|U; the\nreport shows SS|U|W happening as well).  Fix the handling of unknown\nerror codes by dropping SS and (with a WARN) any bits not among those\nthat permission_fault() expects to receive.\n\nAt the same time, given the uncertainty about when hardware sets SS,\nfollow the processor's behavior when constructing the fault's error\ncode, and preserve the SS bit as passed to FNAME(walk_addr_generic).\nWhile the behavior is surprising, there's a possibility that Hyper-V\nexpects it (Hyper-V does not even enable CET unless it finds MBEC/GMET!),\nso retain the information when reflecting the fault to L1 instead of\nunconditionally discarding it.\n\nBisection shows that this started happening with commit 687ee95c1b6d (\"KVM:\nnSVM: enable GMET for guests\"), even though SHSTK was available to guests\nsince Linux 6.18.  However, this is not because bit 1 of MISC_CTL (GMET)\nsomehow enables reporting the SS bit; rather, it's simply that Hyper-V\nnever uses CET unless GMET is also present.  The WARN is reproducible\neven with gmet=0, with some hacking of kvm-unit-tests's cet.c testcase\nto ensure that the first access to the shadow stack triggers a page fault.","Type":"Description","Title":"KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes"}]}}}