{"id":"CVE-2026-63937","state":"PUBLISHED","title":"KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer\n\nUse READ_ONCE() when reading entries/indices from the guest-accessible\nPage State Change buffer to defend against TOCTOU bugs.\n\nDon't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing\n(and not consuming the result!), as the guest isn't supposed to touch the\nbuffer while it's being processed.  I.e. using READ_ONCE() is all about\nprotecting against misbehaving guests.","publishedAt":"2026-07-19T14:55:34.554Z","updatedAt":"2026-10-19T20:17:57.516Z","severity":{"score":8.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"bd232801ef1d1fd985d2d4ca3cd1d888303ca86f","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"b1dfaa6f7a957726a6800135be3659fbe4bbf2a4","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"edbbe88f83b524434974e84808d3093199d67c24","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"c8cc238093ca6c99267032f6cfe78f59389f3157","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kvm/svm/sev.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.11","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.11","versionType":"semver"},{"version":"6.12.93","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.35","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.0.12","status":"unaffected","lessThanOrEqual":"7.0.*","versionType":"semver"},{"version":"7.1","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kvm/svm/sev.c"]}],"references":["https://git.kernel.org/stable/c/bd232801ef1d1fd985d2d4ca3cd1d888303ca86f","https://git.kernel.org/stable/c/b1dfaa6f7a957726a6800135be3659fbe4bbf2a4","https://git.kernel.org/stable/c/edbbe88f83b524434974e84808d3093199d67c24","https://git.kernel.org/stable/c/c8cc238093ca6c99267032f6cfe78f59389f3157"],"weaknesses":["CWE-367"],"ingestedAt":"2026-10-29T20:12:41.951Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/63xxx/CVE-2026-63937.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-63937","state":"PUBLISHED","datePublished":"2026-07-19T14:55:34.554Z","dateUpdated":"2026-10-19T20:17:57.516Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-07-19T07:54:57.022Z"},"containers":{"cna":{"title":"KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer\n\nUse READ_ONCE() when reading entries/indices from the guest-accessible\nPage State Change buffer to defend against TOCTOU bugs.\n\nDon't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing\n(and not consuming the result!), as the guest isn't supposed to touch the\nbuffer while it's being processed.  I.e. using READ_ONCE() is all about\nprotecting against misbehaving guests."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"bd232801ef1d1fd985d2d4ca3cd1d888303ca86f","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"b1dfaa6f7a957726a6800135be3659fbe4bbf2a4","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"edbbe88f83b524434974e84808d3093199d67c24","versionType":"git"},{"version":"9b54e248d2644be71cb394eb85f31ad99e023a05","status":"affected","lessThan":"c8cc238093ca6c99267032f6cfe78f59389f3157","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kvm/svm/sev.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.11","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.11","versionType":"semver"},{"version":"6.12.93","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.35","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.0.12","status":"unaffected","lessThanOrEqual":"7.0.*","versionType":"semver"},{"version":"7.1","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kvm/svm/sev.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/bd232801ef1d1fd985d2d4ca3cd1d888303ca86f"},{"url":"https://git.kernel.org/stable/c/b1dfaa6f7a957726a6800135be3659fbe4bbf2a4"},{"url":"https://git.kernel.org/stable/c/edbbe88f83b524434974e84808d3093199d67c24"},{"url":"https://git.kernel.org/stable/c/c8cc238093ca6c99267032f6cfe78f59389f3157"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H","baseScore":8.8,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:L - Exploitation requires code execution inside an SEV-SNP guest that issues VMGEXIT PSC events into the host KVM hypervisor; this is local access to the host kernel via a guest-controlled hypercall path, not a remote network protocol.\nAC:L - The attacker fully controls the guest-accessible PSC buffer and can trigger concurrent VMGEXITs or use multiple vCPUs to modify indices/entries between KVM's validation read and later use, reliably winning the TOCTOU race without depending on uncontrollable host conditions.\nPR:L - The attacker only needs the ability to run code in an SEV-SNP guest on a KVM host (e.g., a cloud confidential-computing tenant), not real root or capabilities in the host's init namespace.\nUI:N - No victim interaction is required beyond the attacker operating their own malicious guest workload; PSC VMGEXITs are triggered programmatically from guest code.\nS:C - The vulnerable code runs in the host hypervisor but is reached from an untrusted guest; successful exploitation crosses the VM security boundary and can alter host-managed SEV-SNP page-state (RMP) for guest physical addresses.\nC:H - TOCTOU manipulation of PSC entries/indices can bypass validation and drive KVM_HC_MAP_GPA_RANGE on unvalidated GPA ranges, potentially flipping pages between private and shared and disclosing encrypted guest (or host-visible) memory beyond what was checked.\nI:H - The same TOCTOU allows inflating combined page ranges and operations passed to userspace, enabling unauthorized modification of page encryption attributes and corrupting hypervisor-managed memory state via misdirected MAP_GPA_RANGE processing.\nA:H - Incorrect indices and out-of-sync buffer state can cause invalid hypervisor processing, kernel warnings/panics, or repeated disruption of the host's KVM/SEV-SNP path; memory-corruption-class TOCTOU bugs are scored at full availability impact when exploitability is uncertain."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-08-05T12:37:24.499Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.11","versionEndExcluding":"6.12.93"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.11","versionEndExcluding":"6.18.35"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.11","versionEndExcluding":"7.0.12"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.11","versionEndExcluding":"7.1"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-63937","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"total"}],"version":"2.0.3","timestamp":"2026-10-16T15:33:43.123685Z"}}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-367","lang":"en","type":"CWE","description":"CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition"}]}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-19T20:17:57.516Z"}}]}}}