# CVE-2026-63806: KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

> A guest VM can trigger a kernel BUG_ON crash in KVM by performing specific unaligned memory stores to an ioeventfd, causing a denial of service.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-92555, BELL-CVE-2026-63806, DEBIAN-CVE-2026-63806, GHSA-3rc6-xmr7-v8j2, ROOT-OS-DEBIAN-11-CVE-2026-63806, ROOT-OS-DEBIAN-12-CVE-2026-63806, ROOT-OS-UBUNTU-2204-CVE-2026-63806, UBUNTU-CVE-2026-63806

## What is CVE-2026-63806?

CVE-2026-63806 is a high-severity denial of service vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-617 (Reachable Assertion), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-63806?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-63806?

Apply the vendor's patch.

## Is CVE-2026-63806 being exploited?

There is no known exploitation of CVE-2026-63806 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63806 and fix it if it is affected.

What it is: CVE-2026-63806 is a high-severity denial of service vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-63806 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63806/ (Markdown: https://insights.sbomify.com/CVE-2026-63806/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63806
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63806
- OSV: https://osv.dev/vulnerability/CVE-2026-63806

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63806/
Summaries are generated and may contain mistakes; the official description above is the record.