{"id":"AZL-83846","summary":"CVE-2026-31588 affecting package kernel for versions less than 6.6.137.1-1","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":"2026-08-28T17:48:03.422131022Z","published":"2026-04-24T15:16:33Z","upstream":["CVE-2026-31588"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31588"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:3","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.6.137.1-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-83846.json"}}],"schema_version":"1.9.0"}