{"id":"CVE-2026-31588","summary":"KVM: x86: Use scratch field in MMIO fragment to hold small write values","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86: Use scratch field in MMIO fragment to hold small write values\n\nWhen exiting to userspace to service an emulated MMIO write, copy the\nto-be-written value to a scratch field in the MMIO fragment if the size\nof the data payload is 8 bytes or less, i.e. can fit in a single chunk,\ninstead of pointing the fragment directly at the source value.\n\nThis fixes a class of use-after-free bugs that occur when the emulator\ninitiates a write using an on-stack, local variable as the source, the\nwrite splits a page boundary, *and* both pages are MMIO pages.  Because\nKVM's ABI only allows for physically contiguous MMIO requests, accesses\nthat split MMIO pages are separated into two fragments, and are sent to\nuserspace one at a time.  When KVM attempts to complete userspace MMIO in\nresponse to KVM_RUN after the first fragment, KVM will detect the second\nfragment and generate a second userspace exit, and reference the on-stack\nvariable.\n\nThe issue is most visible if the second KVM_RUN is performed by a separate\ntask, in which case the stack of the initiating task can show up as truly\nfreed data.\n\n  ==================================================================\n  BUG: KASAN: use-after-free in complete_emulated_mmio+0x305/0x420\n  Read of size 1 at addr ffff888009c378d1 by task syz-executor417/984\n\n  CPU: 1 PID: 984 Comm: syz-executor417 Not tainted 5.10.0-182.0.0.95.h2627.eulerosv2r13.x86_64 #3\n  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014 Call Trace:\n  dump_stack+0xbe/0xfd\n  print_address_description.constprop.0+0x19/0x170\n  __kasan_report.cold+0x6c/0x84\n  kasan_report+0x3a/0x50\n  check_memory_region+0xfd/0x1f0\n  memcpy+0x20/0x60\n  complete_emulated_mmio+0x305/0x420\n  kvm_arch_vcpu_ioctl_run+0x63f/0x6d0\n  kvm_vcpu_ioctl+0x413/0xb20\n  __se_sys_ioctl+0x111/0x160\n  do_syscall_64+0x30/0x40\n  entry_SYSCALL_64_after_hwframe+0x67/0xd1\n  RIP: 0033:0x42477d\n  Code: \u003c48\u003e 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48\n  RSP: 002b:00007faa8e6890e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010\n  RAX: ffffffffffffffda RBX: 00000000004d7338 RCX: 000000000042477d\n  RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005\n  RBP: 00000000004d7330 R08: 00007fff28d546df R09: 0000000000000000\n  R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004d733c\n  R13: 0000000000000000 R14: 000000000040a200 R15: 00007fff28d54720\n\n  The buggy address belongs to the page:\n  page:0000000029f6a428 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x9c37\n  flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)\n  raw: 000fffffc0000000 0000000000000000 ffffea0000270dc8 0000000000000000\n  raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000 page dumped because: kasan: bad access detected\n\n  Memory state around the buggy address:\n  ffff888009c37780: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff\n  ffff888009c37800: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff\n  \u003effff888009c37880: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff\n                                                   ^\n  ffff888009c37900: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff\n  ffff888009c37980: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff\n  ==================================================================\n\nThe bug can also be reproduced with a targeted KVM-Unit-Test by hacking\nKVM to fill a large on-stack variable in complete_emulated_mmio(), i.e. by\noverwrite the data value with garbage.\n\nLimit the use of the scratch fields to 8-byte or smaller accesses, and to\njust writes, as larger accesses and reads are not affected thanks to\nimplementation details in the emulator, but add a sanity check to ensure\nthose details don't change in the future.  Specifically, KVM never uses\non-stack variables for accesses larger that 8 bytes, e.g. uses an operand\nin the emulator context, and *al\n---truncated---","modified":"2026-06-03T08:44:11.761409781Z","published":"2026-04-24T14:42:16.288Z","related":["CGA-ppv3-26f5-822q","SUSE-SU-2026:2111-1","SUSE-SU-2026:21841-1","SUSE-SU-2026:21845-1","SUSE-SU-2026:21860-1","SUSE-SU-2026:2195-1","SUSE-SU-2026:2202-1","SUSE-SU-2026:2215-1","SUSE-SU-2026:2216-1","SUSE-SU-2026:2217-1","openSUSE-SU-2026:10703-1","openSUSE-SU-2026:20826-1"],"database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31588.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/019d0bd32b9a4646ba35d904907452039e2db700"},{"type":"WEB","url":"https://git.kernel.org/stable/c/0b16e69d17d8c35c5c9d5918bf596c75a44655d3"},{"type":"WEB","url":"https://git.kernel.org/stable/c/22d2ff69d487a32a8b88f9c970120fc2daa08a77"},{"type":"WEB","url":"https://git.kernel.org/stable/c/2b83d91e9ae92fe1258d7040a32430bbb3bb7d6e"},{"type":"WEB","url":"https://git.kernel.org/stable/c/3a7b6d75c8f85b09dea893f64a85a356bcf6c3fe"},{"type":"WEB","url":"https://git.kernel.org/stable/c/4569c66dd9e94a22cd0796b6514a8b25ffff16a1"},{"type":"WEB","url":"https://git.kernel.org/stable/c/52570e73d48f1c73836d37e594667117b4c2a5a8"},{"type":"WEB","url":"https://git.kernel.org/stable/c/b5a02d37eb0739f462fa12df449ab9b3480c783b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/dc6a6c3db3a4eca7e747cfc46e22c08d016c68f7"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31588.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31588"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"f78146b0f9230765c6315b2e14f56112513389ad"},{"fixed":"019d0bd32b9a4646ba35d904907452039e2db700"},{"fixed":"4569c66dd9e94a22cd0796b6514a8b25ffff16a1"},{"fixed":"52570e73d48f1c73836d37e594667117b4c2a5a8"},{"fixed":"dc6a6c3db3a4eca7e747cfc46e22c08d016c68f7"},{"fixed":"b5a02d37eb0739f462fa12df449ab9b3480c783b"},{"fixed":"22d2ff69d487a32a8b88f9c970120fc2daa08a77"},{"fixed":"2b83d91e9ae92fe1258d7040a32430bbb3bb7d6e"},{"fixed":"3a7b6d75c8f85b09dea893f64a85a356bcf6c3fe"},{"fixed":"0b16e69d17d8c35c5c9d5918bf596c75a44655d3"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31588.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.5.0"},{"fixed":"5.10.258"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.209"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.175"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.136"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.83"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.24"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"6.19.14"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.20.0"},{"fixed":"7.0.1"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31588.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"}]}