CVE-2026-63806

In the Linux kernel, the following vulnerability has been resolved:

KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

Drop a BUG_ON() that has been reachable since it was first added, way back

in 2009, and instead use get_unaligned() to perform potentially-unaligned

accesses.

For a given store, KVM x86's emulator tracks the entire value in the

destination operand, x86_emulate_ctxt.dst. If the destination is memory,

and the target splits multiple pages and/or is emulated MMIO, then KVM

handles each fragment independently. E.g. on a page split starting at page

offset 0xffc, KVM writes 4 bytes to the first page, then the remaining

bytes to the second page, using ctxt->dst as the source for both (with

appropriate offsets).

If the destination splits a page *and* hits emulated MMIO on the second

page, then KVM will complete the write to the first page, then emulate the

MMIO access to the second page. If there is a datamatch-enabled ioeventfd

at offset 0 of the second page, then KVM will process the remainder of the

store as a potential ioeventfd signal.

Putting it all together, if the guest emits a store that splits a page

starting at page offset N, and the second page has a datamatch-enabled

ioeventfd at offset 0, then KVM will check for datamatch using

&dst.valptr[N] as the source. Due to dst (and thus dst.valptr) being

32-byte aligned, if N is not aligned to @len, the BUG_ON() fires.

E.g. with a 16-byte store at page offset 0xffc, to an ioeventfd of len 8,

all initial checks in ioeventfd_in_range() will succeed, and the BUG_ON()

fires due to @val being 4-byte aligned, but not 8-byte aligned.

------------[ cut here ]------------

kernel BUG at arch/x86/kvm/../../../virt/kvm/eventfd.c:783!

Oops: invalid opcode: 0000 [#1] SMP

CPU: 0 UID: 1000 PID: 615 Comm: repro Not tainted 7.1.0-rc2-ff238429d1ea #365 PREEMPT

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015

RIP: 0010:ioeventfd_write+0x6c/0x70 [kvm]

Call Trace:

<TASK>

__kvm_io_bus_write+0x85/0xb0 [kvm]

kvm_io_bus_write+0x53/0x80 [kvm]

vcpu_mmio_write+0x66/0xf0 [kvm]

emulator_read_write_onepage+0x12a/0x540 [kvm]

emulator_read_write+0x109/0x2b0 [kvm]

x86_emulate_insn+0x4f8/0xfb0 [kvm]

x86_emulate_instruction+0x181/0x790 [kvm]

kvm_mmu_page_fault+0x313/0x630 [kvm]

vmx_handle_exit+0x18a/0x590 [kvm_intel]

kvm_arch_vcpu_ioctl_run+0xc81/0x1c90 [kvm]

kvm_vcpu_ioctl+0x2d5/0x970 [kvm]

__x64_sys_ioctl+0x8a/0xd0

do_syscall_64+0xb7/0x890

entry_SYSCALL_64_after_hwframe+0x4b/0x53

RIP: 0033:0x7f19c931a9bf

</TASK>

Modules linked in: kvm_intel kvm irqbypass

---[ end trace 0000000000000000 ]---

In a perfect world, the fix would be to simply delete the BUG_ON(), as KVM

x86 doesn't perform alignment checks on "normal" memory accesses at CPL0.

Sadly, C99 ruins all the fun; while the x86 architecture plays nice,

dereferencing an unaligned pointer directly is undefined behavior in C,

e.g. triggers splats when running with CONFIG_UBSAN_ALIGNMENT=y.