CVE-2026-63806 (GCVE-0-2026-63806)
Vulnerability from cvelistv5
Published
2026-07-19 12:02
Modified
2026-08-17 04:51
Severity ?
7.1 (High) - CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned() Drop a BUG_ON() that has been reachable since it was first added, way back in 2009, and instead use get_unaligned() to perform potentially-unaligned accesses. For a given store, KVM x86's emulator tracks the entire value in the destination operand, x86_emulate_ctxt.dst. If the destination is memory, and the target splits multiple pages and/or is emulated MMIO, then KVM handles each fragment independently. E.g. on a page split starting at page offset 0xffc, KVM writes 4 bytes to the first page, then the remaining bytes to the second page, using ctxt->dst as the source for both (with appropriate offsets). If the destination splits a page *and* hits emulated MMIO on the second page, then KVM will complete the write to the first page, then emulate the MMIO access to the second page. If there is a datamatch-enabled ioeventfd at offset 0 of the second page, then KVM will process the remainder of the store as a potential ioeventfd signal. Putting it all together, if the guest emits a store that splits a page starting at page offset N, and the second page has a datamatch-enabled ioeventfd at offset 0, then KVM will check for datamatch using &dst.valptr[N] as the source. Due to dst (and thus dst.valptr) being 32-byte aligned, if N is not aligned to @len, the BUG_ON() fires. E.g. with a 16-byte store at page offset 0xffc, to an ioeventfd of len 8, all initial checks in ioeventfd_in_range() will succeed, and the BUG_ON() fires due to @val being 4-byte aligned, but not 8-byte aligned. ------------[ cut here ]------------ kernel BUG at arch/x86/kvm/../../../virt/kvm/eventfd.c:783! Oops: invalid opcode: 0000 [#1] SMP CPU: 0 UID: 1000 PID: 615 Comm: repro Not tainted 7.1.0-rc2-ff238429d1ea #365 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:ioeventfd_write+0x6c/0x70 [kvm] Call Trace: <TASK> __kvm_io_bus_write+0x85/0xb0 [kvm] kvm_io_bus_write+0x53/0x80 [kvm] vcpu_mmio_write+0x66/0xf0 [kvm] emulator_read_write_onepage+0x12a/0x540 [kvm] emulator_read_write+0x109/0x2b0 [kvm] x86_emulate_insn+0x4f8/0xfb0 [kvm] x86_emulate_instruction+0x181/0x790 [kvm] kvm_mmu_page_fault+0x313/0x630 [kvm] vmx_handle_exit+0x18a/0x590 [kvm_intel] kvm_arch_vcpu_ioctl_run+0xc81/0x1c90 [kvm] kvm_vcpu_ioctl+0x2d5/0x970 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0xb7/0x890 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f19c931a9bf </TASK> Modules linked in: kvm_intel kvm irqbypass ---[ end trace 0000000000000000 ]--- In a perfect world, the fix would be to simply delete the BUG_ON(), as KVM x86 doesn't perform alignment checks on "normal" memory accesses at CPL0. Sadly, C99 ruins all the fun; while the x86 architecture plays nice, dereferencing an unaligned pointer directly is undefined behavior in C, e.g. triggers splats when running with CONFIG_UBSAN_ALIGNMENT=y.
References
Impacted products
Vendor Product Version
Linux Linux Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
Version: d34e6b175e61821026893ec5298cc8e7558df43a
 Create a notification for this product.
Show details on NVD website

To clipboard 

{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "virt/kvm/eventfd.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "2426c15c1395b7d5ccf1e5025ca898af7f3decb6",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "4186c850789906b875a1d263377a4d37c078e317",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "36ff44fb3d89960391e013fb9d91e23dbc48be47",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "92fc631b69deb1c7d56aec2663003600799dcd75",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "bf89e3738480d33cd515b4a18900e8443d40cd2e",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "5da9b1a87ec7cc3489c27016313524769f12d9e0",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "5c87b47374682f69686068ad0a7779365a527b1c",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "f1edbed787ba67988ed34e0132ca128b052b6ce8",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "virt/kvm/eventfd.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "2.6.32"
            },
            {
              "lessThan": "2.6.32",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.10.*",
              "status": "unaffected",
              "version": "5.10.261",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.15.*",
              "status": "unaffected",
              "version": "5.15.212",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.1.*",
              "status": "unaffected",
              "version": "6.1.178",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.6.*",
              "status": "unaffected",
              "version": "6.6.145",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.95",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.38",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.3",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "5.10.261",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "5.15.212",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.1.178",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.6.145",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.12.95",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.18.38",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.1.3",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2",
                  "versionStartIncluding": "2.6.32",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()\n\nDrop a BUG_ON() that has been reachable since it was first added, way back\nin 2009, and instead use get_unaligned() to perform potentially-unaligned\naccesses.\n\nFor a given store, KVM x86\u0027s emulator tracks the entire value in the\ndestination operand, x86_emulate_ctxt.dst.  If the destination is memory,\nand the target splits multiple pages and/or is emulated MMIO, then KVM\nhandles each fragment independently.  E.g. on a page split starting at page\noffset 0xffc, KVM writes 4 bytes to the first page, then the remaining\nbytes to the second page, using ctxt-\u003edst as the source for both (with\nappropriate offsets).\n\nIf the destination splits a page *and* hits emulated MMIO on the second\npage, then KVM will complete the write to the first page, then emulate the\nMMIO access to the second page.  If there is a datamatch-enabled ioeventfd\nat offset 0 of the second page, then KVM will process the remainder of the\nstore as a potential ioeventfd signal.\n\nPutting it all together, if the guest emits a store that splits a page\nstarting at page offset N, and the second page has a datamatch-enabled\nioeventfd at offset 0, then KVM will check for datamatch using\n\u0026dst.valptr[N] as the source.  Due to dst (and thus dst.valptr) being\n32-byte aligned, if N is not aligned to @len, the BUG_ON() fires.\n\nE.g. with a 16-byte store at page offset 0xffc, to an ioeventfd of len 8,\nall initial checks in ioeventfd_in_range() will succeed, and the BUG_ON()\nfires due to @val being 4-byte aligned, but not 8-byte aligned.\n\n  ------------[ cut here ]------------\n  kernel BUG at arch/x86/kvm/../../../virt/kvm/eventfd.c:783!\n  Oops: invalid opcode: 0000 [#1] SMP\n  CPU: 0 UID: 1000 PID: 615 Comm: repro Not tainted 7.1.0-rc2-ff238429d1ea #365 PREEMPT\n  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n  RIP: 0010:ioeventfd_write+0x6c/0x70 [kvm]\n  Call Trace:\n   \u003cTASK\u003e\n   __kvm_io_bus_write+0x85/0xb0 [kvm]\n   kvm_io_bus_write+0x53/0x80 [kvm]\n   vcpu_mmio_write+0x66/0xf0 [kvm]\n   emulator_read_write_onepage+0x12a/0x540 [kvm]\n   emulator_read_write+0x109/0x2b0 [kvm]\n   x86_emulate_insn+0x4f8/0xfb0 [kvm]\n   x86_emulate_instruction+0x181/0x790 [kvm]\n   kvm_mmu_page_fault+0x313/0x630 [kvm]\n   vmx_handle_exit+0x18a/0x590 [kvm_intel]\n   kvm_arch_vcpu_ioctl_run+0xc81/0x1c90 [kvm]\n   kvm_vcpu_ioctl+0x2d5/0x970 [kvm]\n   __x64_sys_ioctl+0x8a/0xd0\n   do_syscall_64+0xb7/0x890\n   entry_SYSCALL_64_after_hwframe+0x4b/0x53\n  RIP: 0033:0x7f19c931a9bf\n   \u003c/TASK\u003e\n  Modules linked in: kvm_intel kvm irqbypass\n  ---[ end trace 0000000000000000 ]---\n\nIn a perfect world, the fix would be to simply delete the BUG_ON(), as KVM\nx86 doesn\u0027t perform alignment checks on \"normal\" memory accesses at CPL0.\nSadly, C99 ruins all the fun; while the x86 architecture plays nice,\ndereferencing an unaligned pointer directly is undefined behavior in C,\ne.g. triggers splats when running with CONFIG_UBSAN_ALIGNMENT=y."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.1,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - A malicious KVM guest triggers this via guest-executed MMIO stores handled by the x86 instruction emulator (KVM_RUN \u2192 vmx_handle_exit \u2192 kvm_mmu_page_fault \u2192 x86_emulate_instruction \u2192 emulator_read_write \u2192 vcpu_mmio_write \u2192 ioeventfd_write), not via remote network input to the host.\nAC:L - The guest fully controls the spanning store address, size, and timing; with standard QEMU/virtio datamatch ioeventfd MMIO layouts, a page-boundary write reliably reaches ioeventfd_in_range() with a misaligned val pointer and fires the BUG_ON().\nPR:N - Exploitation requires only unprivileged code execution inside an already-running guest VM (reproducer ran as UID 1000); no host capabilities, KVM ioctls, or guest root are needed beyond what any cloud VM tenant or compromised guest process already has.\nUI:N - No victim interaction is required once the attacker can run code in a KVM guest with a datamatch ioeventfd configured, which is the default virtio/QEMU setup.\nS:C - The vulnerable component is the host kernel KVM subsystem, but exploitation is initiated from a guest VM and causes a host kernel BUG/oops, crossing the hypervisor security boundary.\nC:N - The failure mode is an explicit BUG_ON() alignment check before any datamatch comparison; there is no out-of-bounds read, use-after-free, or other memory corruption that could disclose host data.\nI:N - The bug triggers a deliberate kernel BUG and does not corrupt or modify host memory; it is a crash-only denial-of-service with no integrity impact.\nA:H - A guest-triggered BUG_ON() in ioeventfd_write() causes a host kernel oops/panic, denying availability of the entire physical host and all co-resident VMs on multi-tenant KVM hypervisors."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-17T04:51:12.196Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/2426c15c1395b7d5ccf1e5025ca898af7f3decb6"
        },
        {
          "url": "https://git.kernel.org/stable/c/4186c850789906b875a1d263377a4d37c078e317"
        },
        {
          "url": "https://git.kernel.org/stable/c/36ff44fb3d89960391e013fb9d91e23dbc48be47"
        },
        {
          "url": "https://git.kernel.org/stable/c/92fc631b69deb1c7d56aec2663003600799dcd75"
        },
        {
          "url": "https://git.kernel.org/stable/c/bf89e3738480d33cd515b4a18900e8443d40cd2e"
        },
        {
          "url": "https://git.kernel.org/stable/c/5da9b1a87ec7cc3489c27016313524769f12d9e0"
        },
        {
          "url": "https://git.kernel.org/stable/c/5c87b47374682f69686068ad0a7779365a527b1c"
        },
        {
          "url": "https://git.kernel.org/stable/c/f1edbed787ba67988ed34e0132ca128b052b6ce8"
        }
      ],
      "title": "KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63806",
    "datePublished": "2026-07-19T12:02:10.209Z",
    "dateReserved": "2026-07-19T07:54:57.013Z",
    "dateUpdated": "2026-08-17T04:51:12.196Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


Log in or create an account to share your comment.




Tags
Taxonomy of the tags.




Sightings

Author Source Type Date

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
  • Confirmed: The vulnerability is confirmed from an analyst perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
  • Patched: This vulnerability was successfully patched by the user reporting the sighting.
  • Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
  • Not confirmed: The user expresses doubt about the veracity of the vulnerability.
  • Not patched: This vulnerability was not successfully patched by the user reporting the sighting.