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

Overview

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.

Affected

• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 2028b81321dc757b6875b99c10d908e349c141e2
• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 60283726f2845bd78b95efbd0e50b93944780477
• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 7508916b4b55d6f5ecc68cd09774dabd3a6b4440
• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 0f33b1c457c2199ed130b92cc2ff363a3f7b9415
• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 6b542d116acecb83a1ca34e8eace304cff6a4ec9
• Linux >= 774c47f1d78e373a6bd2964f4e278d1ce26c21cb < 25f744ffa0c8e799e06250ce2e618367b166b0d4
• Linux 2.6.21

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.