CVE-2026-63939 (GCVE-0-2026-63939)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
VLAI
Title
KVM: SEV: Compute the correct max length of the in-GHCB scratch area
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Compute the correct max length of the in-GHCB scratch area When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer. The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required. But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert). Opportunistically rename @len => @min_len to better reflect its role.
Severity
9.3 (Critical)
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < 6ca9400d36005ffdca25f80186bea781c7e1dc4c (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < 9f0a9e780f02c02d025a190f1885e1d1d73b87bd (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < 6644565527c4c5f507088b1c9ddf72de47790b68 (git)
Affected: 9b54e248d2644be71cb394eb85f31ad99e023a05 , < 5867d7e202e09f037cefe77f7af4413c7c0fa088 (git)
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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)
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{
  "containers": {
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      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "arch/x86/kvm/svm/sev.c"
          ],
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: SEV: Compute the correct max length of the in-GHCB scratch area\n\nWhen setting the length of the GHCB scratch area, and the area is in the\nGHCB shared buffer, set the effective length of the scratch area to the max\npossible size given the start of the guest-provided pointer, and the end of\nthe shared buffer.\n\nThe code was \"fine\" when first introduced, as KVM doesn\u0027t consult the\nlength of the buffer when emulating MMIO, because the passed in @len always\nspecifies the *max* size required.  But for PSC requests, the incoming @len\nis just the minimum length (to process the header), and KVM needs to know\nthe full size of the scratch area to avoid buffer overflows (spoiler alert).\n\nOpportunistically rename @len =\u003e @min_len to better reflect its role."
        }
      ],
      "metrics": [
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            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - Exploitation requires code running inside an SEV-SNP KVM guest that issues a PSC VMGEXIT via the GHCB protocol; the vulnerable path is reached through the local VM/hypercall interface (SVM_EXIT_VMGEXIT), not via any network-facing protocol.\nAC:L - Once running in an SEV-SNP guest, the attacker fully controls the GHCB scratch GPA, PSC header indices, and entry contents, and can reliably trigger the bug without races or conditions outside their control.\nPR:N - No host privileges are required; a cloud tenant or any party able to run a malicious SEV-SNP guest VM can reach this code without host root or CAP_SYS_ADMIN in the host init namespace.\nUI:N - Exploitation requires no action from a victim user; the malicious guest triggers the vulnerability autonomously during PSC VMGEXIT processing.\nS:C - The flaw is in KVM host-side handling of guest VMGEXIT requests and enables out-of-bounds kernel access that crosses the guest-to-hypervisor security boundary, consistent with a VM escape primitive.\nC:H - The bug causes out-of-bounds reads of PSC entry structures beyond the computed scratch buffer length, which can leak kernel-mapped memory adjacent to the GHCB page.\nI:H - The kernel writes PSC completion data (entry cur_page and hdr.cur_entry) using attacker-controlled indices beyond the actual scratch buffer, enabling out-of-bounds writes exploitable for memory corruption.\nA:H - Out-of-bounds access within the host\u0027s GHCB mapping can corrupt adjacent memory and trigger kernel oopses or panics, and repeated exploitation can deny service to the host and other tenants."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:37:26.628Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
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          "url": "https://git.kernel.org/stable/c/6ca9400d36005ffdca25f80186bea781c7e1dc4c"
        },
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        },
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        },
        {
          "url": "https://git.kernel.org/stable/c/5867d7e202e09f037cefe77f7af4413c7c0fa088"
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      ],
      "title": "KVM: SEV: Compute the correct max length of the in-GHCB scratch area",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63939",
    "datePublished": "2026-07-19T14:55:35.743Z",
    "dateReserved": "2026-07-19T07:54:57.022Z",
    "dateUpdated": "2026-08-05T12:37:26.628Z",
    "state": "PUBLISHED"
  },
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      "percentile": "0.09661"
    },
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    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-07-21T22:01:01+00:00",
      "cve": "CVE-2026-63939",
      "id": "CVE-2026-63939",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "184",
      "product_status:known_not_affected": "91",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: KVM: SEV: Compute the correct max length of the in-GHCB scratch area",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63939.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-08-10T16:25:10Z",
      "cve": "CVE-2026-63939",
      "id": "CVE-2026-63939",
      "initial_release_date": "2026-07-21T13:03:07Z",
      "product_status:first_fixed": "98",
      "product_status:known_affected": "55",
      "product_status:known_not_affected": "229",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-63939",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-63939.json",
      "version": "6"
    }
  }
}



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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

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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.