CVE-2026-68093: KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug

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

KVM: SVM: Bump asidgeneration 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.

svmenablevirtualizationcpu() resets asidgeneration to 1 and sets nextasid to maxasid + 1 on every CPU online event, including hotplug cycles. Because nextasid starts beyond the pool boundary, the first call to newasid() after an online event always wraps the pool, incrementing asidgeneration to 2 and assigning ASIDs starting from minasid.

Consider two vCPUs from different VMs, vCPU-A pinned to CPU-X holding asidgeneration=2 and ASID=N from before the hotplug event:

1. CPU-X goes offline and back online: asidgeneration resets to 1, nextasid = maxasid + 1.

2. One or more vCPUs migrate to CPU-X and call newasid(), wrapping the pool and consuming ASIDs starting from minasid. Eventually vCPU-B from a different VM is assigned asidgeneration=2, ASID=N — the same ASID that vCPU-A held before the hotplug.

3. vCPU-A enters presvmrun() on CPU-X: currentvmcb->cpu is unchanged so the migration branch is skipped. Its saved asidgeneration=2 matches sd->asidgeneration=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 asidgeneration instead of resetting it to 1 in svmenablevirtualizationcpu(). On module load, asidgeneration 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 presvmrun() reliably forces newasid() on every vCPU after every hotplug cycle.