Vulnerability-Lookup

GHSA-3RC6-XMR7-V8J2

Vulnerability from github – Published: 2026-07-19 12:30 – Updated: 2026-07-20 15:31

Details

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: __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 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.

Severity

7.1 (High)


                  
                    CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

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{
  "affected": [],
  "aliases": [
    "CVE-2026-63806"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-07-19T12:16:53Z",
    "severity": "HIGH"
  },
  "details": "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.",
  "id": "GHSA-3rc6-xmr7-v8j2",
  "modified": "2026-07-20T15:31:47Z",
  "published": "2026-07-19T12:30:23Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63806"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5c87b47374682f69686068ad0a7779365a527b1c"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5da9b1a87ec7cc3489c27016313524769f12d9e0"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/bf89e3738480d33cd515b4a18900e8443d40cd2e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/f1edbed787ba67988ed34e0132ca128b052b6ce8"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}

CVE-2026-63806 (GCVE-0-2026-63806)

Vulnerability from cvelistv5 – Published: 2026-07-19 12:02 – Updated: 2026-07-20 13:40

Title

KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

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.

Severity

7.1 (High)


                        
                          CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Assigner

Linux

References

4 references

URL	Tags
https://git.kernel.org/stable/c/bf89e3738480d33cd…
https://git.kernel.org/stable/c/5da9b1a87ec7cc348…
https://git.kernel.org/stable/c/5c87b47374682f696…
https://git.kernel.org/stable/c/f1edbed787ba67988…

Impacted products

2 products

Vendor	Product	Version
Linux	Linux	Affected: d34e6b175e61821026893ec5298cc8e7558df43a , < bf89e3738480d33cd515b4a18900e8443d40cd2e (git) Affected: d34e6b175e61821026893ec5298cc8e7558df43a , < 5da9b1a87ec7cc3489c27016313524769f12d9e0 (git) Affected: d34e6b175e61821026893ec5298cc8e7558df43a , < 5c87b47374682f69686068ad0a7779365a527b1c (git) Affected: d34e6b175e61821026893ec5298cc8e7558df43a , < f1edbed787ba67988ed34e0132ca128b052b6ce8 (git)
Linux	Linux	Affected: 2.6.32 Unaffected: 0 , < 2.6.32 (semver) Unaffected: 6.12.95 , ≤ 6.12.* (semver) Unaffected: 6.18.38 , ≤ 6.18.* (semver) Unaffected: 7.1.3 , ≤ 7.1.* (semver) Unaffected: 7.2-rc1 , ≤ * (original_commit_for_fix)

Show details on NVD website

JSON

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{
  "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": "bf89e3738480d33cd515b4a18900e8443d40cd2e",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "5da9b1a87ec7cc3489c27016313524769f12d9e0",
              "status": "affected",
              "version": "d34e6b175e61821026893ec5298cc8e7558df43a",
              "versionType": "git"
            },
            {
              "lessThan": "5c87b47374682f69686068ad0a7779365a527b1c",
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              "versionType": "git"
            },
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              "lessThan": "f1edbed787ba67988ed34e0132ca128b052b6ce8",
              "status": "affected",
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              "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"
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            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
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              "versionType": "semver"
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              "lessThanOrEqual": "6.18.*",
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              "version": "6.18.38",
              "versionType": "semver"
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              "version": "7.1.3",
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            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2-rc1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "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-rc1",
                  "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"
          }
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-20T13:40:15.599Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "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": "2026-07-20T13:40:15.599Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

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.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

GHSA-3RC6-XMR7-V8J2

CVE-2026-63806 (GCVE-0-2026-63806)

Tags

Sightings

Nomenclature