CVE-2026-68093

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

KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug

If a vCPU stays scheduled out (or blocked) while the last pCPU it ran

on goes through a hotplug cycle (online->offline->online), and the vCPU

then resumes execution on the same pCPU, then it is possible for it to

run with an ASID that has now been assigned to a different vCPU,

resulting in stale TLB translations being used.

svm_enable_virtualization_cpu() resets asid_generation to 1 and sets

next_asid to max_asid + 1 on every CPU online event, including hotplug

cycles. Because next_asid starts beyond the pool boundary, the first

call to new_asid() after an online event always wraps the pool,

incrementing asid_generation to 2 and assigning ASIDs starting from

min_asid.

Consider two vCPUs from different VMs, vCPU-A pinned to CPU-X holding

asid_generation=2 and ASID=N from before the hotplug event:

1. CPU-X goes offline and back online: asid_generation resets to 1,

next_asid = max_asid + 1.

2. One or more vCPUs migrate to CPU-X and call new_asid(), wrapping

the pool and consuming ASIDs starting from min_asid. Eventually

vCPU-B from a different VM is assigned asid_generation=2, ASID=N

— the same ASID that vCPU-A held before the hotplug.

3. vCPU-A enters pre_svm_run() on CPU-X: current_vmcb->cpu is

unchanged so the migration branch is skipped. Its saved

asid_generation=2 matches sd->asid_generation=2, so the generation

check silently passes and vCPU-A continues running with ASID=N —

the same ASID just freshly assigned to vCPU-B.

Both vCPUs from different VMs now run on CPU-X with the same ASID,

causing them to share NPT TLB entries and producing stale translations.

The collision manifests as a KVM internal error (Suberror: 1, emulation

failure). The NPT page fault reports a faulting GPA far outside the

VM's physical memory range — a sign of stale TLB translations being

used. KVM falls back to instruction emulation, which fails on

FPU/XSave instructions (XRSTOR, STMXCSR) that the emulator does not

implement.

Fix this by incrementing asid_generation instead of resetting it to 1

in svm_enable_virtualization_cpu(). On module load, asid_generation

starts at 0 (memset) and the increment produces 1, identical to the

old behaviour. On subsequent hotplug cycles the generation advances

beyond any value a vCPU previously observed on this CPU, so the

generation check in pre_svm_run() reliably forces new_asid() on every

vCPU after every hotplug cycle.