{"id":"CVE-2026-63806","revision":"64e7814a5d8d447cb2890f1479cfc9dc","records":[{"revision":"64e7814a5d8d447cb2890f1479cfc9dc","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-63806","aliases":[],"title":"KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()","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 \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 \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.","state":0,"publishedAt":"2026-07-19T12:02:10.209+00:00","modifiedAt":"2026-10-19T20:07:48.133+00:00","affected":[{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"unaffected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"virt/kvm/eventfd.c\"\n ],\n \"versions\": [\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"2426c15c1395b7d5ccf1e5025ca898af7f3decb6\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"4186c850789906b875a1d263377a4d37c078e317\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"36ff44fb3d89960391e013fb9d91e23dbc48be47\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"92fc631b69deb1c7d56aec2663003600799dcd75\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"bf89e3738480d33cd515b4a18900e8443d40cd2e\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"5da9b1a87ec7cc3489c27016313524769f12d9e0\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"5c87b47374682f69686068ad0a7779365a527b1c\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"d34e6b175e61821026893ec5298cc8e7558df43a\",\n \"lessThan\": \"f1edbed787ba67988ed34e0132ca128b052b6ce8\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n }\n ]\n }"},{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"affected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"virt/kvm/eventfd.c\"\n ],\n \"versions\": [\n {\n \"version\": \"2.6.32\",\n \"status\": \"affected\"\n },\n {\n \"version\": \"0\",\n \"lessThan\": \"2.6.32\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.10.261\",\n \"lessThanOrEqual\": \"5.10.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.15.212\",\n \"lessThanOrEqual\": \"5.15.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.1.178\",\n \"lessThanOrEqual\": \"6.1.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.6.145\",\n \"lessThanOrEqual\": \"6.6.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.12.95\",\n \"lessThanOrEqual\": \"6.12.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.18.38\",\n \"lessThanOrEqual\": \"6.18.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.1.3\",\n \"lessThanOrEqual\": \"7.1.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.2\",\n \"lessThanOrEqual\": \"*\",\n \"status\": \"unaffected\",\n \"versionType\": \"original_commit_for_fix\"\n }\n ]\n }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H","score":7.1,"severity":"HIGH"}],"references":["https://git.kernel.org/stable/c/2426c15c1395b7d5ccf1e5025ca898af7f3decb6","https://git.kernel.org/stable/c/4186c850789906b875a1d263377a4d37c078e317","https://git.kernel.org/stable/c/36ff44fb3d89960391e013fb9d91e23dbc48be47","https://git.kernel.org/stable/c/92fc631b69deb1c7d56aec2663003600799dcd75","https://git.kernel.org/stable/c/bf89e3738480d33cd515b4a18900e8443d40cd2e","https://git.kernel.org/stable/c/5da9b1a87ec7cc3489c27016313524769f12d9e0","https://git.kernel.org/stable/c/5c87b47374682f69686068ad0a7779365a527b1c","https://git.kernel.org/stable/c/f1edbed787ba67988ed34e0132ca128b052b6ce8"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-63806","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-63806","identifiers":["CVE-2026-63806"]},"retained":false,"historicalAffected":[]}],"coverage":[]}