CVE-2026-31588: KVM: x86: Use scratch field in MMIO fragment to hold small write values

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. 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 KVMRUN 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 KVMRUN is performed by a separate task, in which case the stack of the initiating task can show up as truly freed data.

================================================================== BUG: KASAN: use-after-free in completeemulatedmmio+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.x8664 #3 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014 Call Trace: dumpstack+0xbe/0xfd printaddressdescription.constprop.0+0x19/0x170 kasanreport.cold+0x6c/0x84 kasanreport+0x3a/0x50 checkmemoryregion+0xfd/0x1f0 memcpy+0x20/0x60 completeemulatedmmio+0x305/0x420 kvmarchvcpuioctlrun+0x63f/0x6d0 kvmvcpuioctl+0x413/0xb20 sesysioctl+0x111/0x160 dosyscall64+0x30/0x40 entrySYSCALL64afterhwframe+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 ORIGRAX: 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 ==================================================================

The bug can also be reproduced with a targeted KVM-Unit-Test by hacking KVM to fill a large on-stack variable in completeemulatedmmio(), 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---