CVE-2026-63937 (GCVE-0-2026-63937)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
VLAI
Title
KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer Use READ_ONCE() when reading entries/indices from the guest-accessible Page State Change buffer to defend against TOCTOU bugs. Don't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing (and not consuming the result!), as the guest isn't supposed to touch the buffer while it's being processed. I.e. using READ_ONCE() is all about protecting against misbehaving guests.
Severity
8.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < bd232801ef1d1fd985d2d4ca3cd1d888303ca86f (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < b1dfaa6f7a957726a6800135be3659fbe4bbf2a4 (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < edbbe88f83b524434974e84808d3093199d67c24 (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < c8cc238093ca6c99267032f6cfe78f59389f3157 (git)
guessed Create a notification for this product.
Linux Linux Affected: 6.11
Unaffected: 0 , < 6.11 (semver)
Unaffected: 6.12.93 , ≤ 6.12.* (semver)
Unaffected: 6.18.35 , ≤ 6.18.* (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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{
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          "product": "Linux",
          "programFiles": [
            "arch/x86/kvm/svm/sev.c"
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              "version": "0",
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              "versionType": "semver"
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              "versionType": "semver"
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          "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\u0027t bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing\n(and not consuming the result!), as the guest isn\u0027t supposed to touch the\nbuffer while it\u0027s being processed.  I.e. using READ_ONCE() is all about\nprotecting against misbehaving guests."
        }
      ],
      "metrics": [
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            "baseScore": 8.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
            "version": "3.1"
          },
          "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\u0027s 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\u0027s 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\u0027s KVM/SEV-SNP path; memory-corruption-class TOCTOU bugs are scored at full availability impact when exploitability is uncertain."
            }
          ]
        }
      ],
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        "dateUpdated": "2026-08-05T12:37:24.499Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
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        },
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        },
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    "cveId": "CVE-2026-63937",
    "datePublished": "2026-07-19T14:55:34.554Z",
    "dateReserved": "2026-07-19T07:54:57.022Z",
    "dateUpdated": "2026-08-05T12:37:24.499Z",
    "state": "PUBLISHED"
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      "date": "2026-09-21",
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            "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\u0027t bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing\n(and not consuming the result!), as the guest isn\u0027t supposed to touch the\nbuffer while it\u0027s being processed.  I.e. using READ_ONCE() is all about\nprotecting against misbehaving guests."
          }
        ],
        "id": "CVE-2026-63937",
        "lastModified": "2026-07-27T17:44:23.777",
        "metrics": {
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        "vulnStatus": "Awaiting Analysis"
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    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-07-22T03:09:29+00:00",
      "cve": "CVE-2026-63937",
      "id": "CVE-2026-63937",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "184",
      "product_status:known_not_affected": "90",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63937.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-09-21T16:49:19Z",
      "cve": "CVE-2026-63937",
      "id": "CVE-2026-63937",
      "initial_release_date": "2026-07-21T13:03:10Z",
      "product_status:first_fixed": "2",
      "product_status:known_affected": "394",
      "product_status:known_not_affected": "342",
      "product_status:recommended": "228",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-63937",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-63937.json",
      "version": "9"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


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