{"id":"CVE-2026-63806","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-63806","generatedAt":"2026-10-28T03:55:31.884Z","title":"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…","description":"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's 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->dst 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&dst.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   <TASK>\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   </TASK>\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't 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.","published":"2026-07-19T12:16:53.000Z","lastModified":"2026-10-19T20:19:21.000Z","status":"Modified","sourceIdentifier":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","cvss":{"version":"3.1","score":7.1,"severity":"HIGH","vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H"},"cwe":["CWE-617","CWE-400"],"affected":["linux/linux kernel"],"epss":{"score":0.00156,"percentile":0.04657,"date":"2026-10-27","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (0.16%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[],"references":[{"url":"https://git.kernel.org/stable/c/2426c15c1395b7d5ccf1e5025ca898af7f3decb6","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/36ff44fb3d89960391e013fb9d91e23dbc48be47","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/4186c850789906b875a1d263377a4d37c078e317","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/5c87b47374682f69686068ad0a7779365a527b1c","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/5da9b1a87ec7cc3489c27016313524769f12d9e0","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/92fc631b69deb1c7d56aec2663003600799dcd75","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/bf89e3738480d33cd515b4a18900e8443d40cd2e","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/f1edbed787ba67988ed34e0132ca128b052b6ce8","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-63806","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}