CVE-2026-31588
Estado: ModificadaAlta (8.8)—
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: Use scratch field in MMIO fragment to hold small write values
When exiting to userspace to service an emulated MMIO write, copy the to-be-written value to a scratch field in the MMIO fragment if the size of the data payload is 8 bytes or less, i.e. can fit in a single chunk, instead of pointing the fragment directly at the source value.
This fixes a class of use-after-free bugs that occur when the emulator initiates a write using an on-stack, local variable as the source, the write splits a page boundary, *and* both pages are MMIO pages.
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Because KVM's ABI only allows for physically contiguous MMIO requests, accesses that split MMIO pages are separated into two fragments, and are sent to userspace one at a time. When KVM attempts to complete userspace MMIO in response to KVM_RUN after the first fragment, KVM will detect the second fragment and generate a second userspace exit, and reference the on-stack variable.
The issue is most visible if the second KVM_RUN is performed by a separate task, in which case the stack of the initiating task can show up as truly freed data.
The bug can also be reproduced with a targeted KVM-Unit-Test by hacking KVM to fill a large on-stack variable in complete_emulated_mmio(), i.e. by overwrite the data value with garbage.
Limit the use of the scratch fields to 8-byte or smaller accesses, and to just writes, as larger accesses and reads are not affected thanks to implementation details in the emulator, but add a sanity check to ensure those details don't change in the future. Specifically, KVM never uses on-stack variables for accesses larger that 8 bytes, e.g. uses an operand in the emulator context, and *al ---truncated---
Detalles técnicos trazas, registros y código del informe original
==================================================================
BUG: KASAN: use-after-free in complete_emulated_mmio+0x305/0x420
Read of size 1 at addr ffff888009c378d1 by task syz-executor417/984
CPU: 1 PID: 984 Comm: syz-executor417 Not tainted 5.10.0-182.0.0.95.h2627.eulerosv2r13.x86_64 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014 Call Trace:
dump_stack+0xbe/0xfd
print_address_description.constprop.0+0x19/0x170
__kasan_report.cold+0x6c/0x84
kasan_report+0x3a/0x50
check_memory_region+0xfd/0x1f0
memcpy+0x20/0x60
complete_emulated_mmio+0x305/0x420
kvm_arch_vcpu_ioctl_run+0x63f/0x6d0
kvm_vcpu_ioctl+0x413/0xb20
__se_sys_ioctl+0x111/0x160
do_syscall_64+0x30/0x40
entry_SYSCALL_64_after_hwframe+0x67/0xd1
RIP: 0033:0x42477d
Code: <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007faa8e6890e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004d7338 RCX: 000000000042477d
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
RBP: 00000000004d7330 R08: 00007fff28d546df R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004d733c
R13: 0000000000000000 R14: 000000000040a200 R15: 00007fff28d54720
The buggy address belongs to the page:
page:0000000029f6a428 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x9c37
flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)
raw: 000fffffc0000000 0000000000000000 ffffea0000270dc8 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000 page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff888009c37780: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888009c37800: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888009c37880: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff888009c37900: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888009c37980: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================CVSS
- Versión: 3.1
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- Puntuación base: 8.8
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.26%
- Percentil entre todas las CVEs puntuadas: 17
- Fecha de la puntuación: 6/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
🎯 Técnicas ATT&CK
Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.
- Explotación
T1068Exploitation for Privilege Escalationprivilege escalation85 % - Impacto principal
T1059Command and Scripting Interpreterexecution75 %
Vulnerabilidad de use-after-free (CWE-416) en KVM que requiere acceso local con privilegios (PR:L, AV:L) para explotar mediante MMIO emulado. Permite ejecución de código en el contexto del hipervisor.
Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.
🛡️ Mitigaciones ATT&CK que cubren estas técnicas
Tecnologías afectadas (1)
CWE
- CWE-416
Referencias
- https://git.kernel.org/stable/c/019d0bd32b9a4646ba35d904907452039e2db700
- https://git.kernel.org/stable/c/0b16e69d17d8c35c5c9d5918bf596c75a44655d3
- https://git.kernel.org/stable/c/22d2ff69d487a32a8b88f9c970120fc2daa08a77
- https://git.kernel.org/stable/c/2b83d91e9ae92fe1258d7040a32430bbb3bb7d6e
- https://git.kernel.org/stable/c/3a7b6d75c8f85b09dea893f64a85a356bcf6c3fe
- https://git.kernel.org/stable/c/4569c66dd9e94a22cd0796b6514a8b25ffff16a1
- https://git.kernel.org/stable/c/52570e73d48f1c73836d37e594667117b4c2a5a8
- https://git.kernel.org/stable/c/b5a02d37eb0739f462fa12df449ab9b3480c783b
- https://git.kernel.org/stable/c/dc6a6c3db3a4eca7e747cfc46e22c08d016c68f7
JSON original (NVD)
Mostrar
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"source": "nvd@nist.gov",
"description": [
{
"lang": "en",
"value": "CWE-416"
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"descriptions": [
{
"lang": "en",
"value": "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: <48> 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 >ffff888009c37880: 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---"
}
],
"lastModified": "2026-06-17T10:34:02.230",
"configurations": [
{
"nodes": [
{
"negate": false,
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}