CVE-2024-26992

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

KVM: x86/pmu: Disable support for adaptive PEBS

Drop support for virtualizing adaptive PEBS, as KVMs implementation is architecturally broken without an obvious/easy path forward, and because exposing adaptive PEBS can leak host LBRs to the guest, i.e. can leak host kernel addresses to the guest.

Bug #1 is that KVM doesnt account for the upper 32 bits of IA32_FIXED_CTR_CTRL when (re)programming fixed counters, e.g fixed_ctrl_field() drops the upper bits, reprogram_fixed_counters() stores local variables as u8s and truncates the upper bits too, etc.

Bug #2 is that, because KVM always sets precise_ip to a non-zero value for PEBS events, perf will always generate an adaptive record, even if the guest requested a basic record. Note, KVM will also enable adaptive PEBS in individual counter, even if adaptive PEBS isnt exposed to the guest, but this is benign as MSR_PEBS_DATA_CFG is guaranteed to be zero, i.e. the guest will only ever see Basic records.

Bug #3 is in perf. intel_pmu_disable_fixed() doesnt clear the upper bits either, i.e. leaves ICL_FIXED_0_ADAPTIVE set, and intel_pmu_enable_fixed() effectively doesnt clear ICL_FIXED_0_ADAPTIVE either. I.e. perf always enables ADAPTIVE counters, regardless of what KVM requests.

Bug #4 is that adaptive PEBS might effectively bypass event filters set by the host, as Updated Memory Access Info Group records information that might be disallowed by userspace via KVM_SET_PMU_EVENT_FILTER.

Bug #5 is that KVM doesnt ensure LBR MSRs hold guest values (or at least zeros) when entering a vCPU with adaptive PEBS, which allows the guest to read host LBRs, i.e. host RIPs/addresses, by enabling LBR Entries records.

Disable adaptive PEBS support as an immediate fix due to the severity of the LBR leak in particular, and because fixing all of the bugs will be non-trivial, e.g. not suitable for backporting to stable kernels.

Note! This will break live migration, but trying to make KVM play nice with live migration would be quite complicated, wouldnt be guaranteed to work (i.e. KVM might still kill/confuse the guest), and its not clear that there are any publicly available VMMs that support adaptive PEBS, let alone live migrate VMs that support adaptive PEBS, e.g. QEMU doesnt support PEBS in any capacity.

Affected Software

References

• https://git.kernel.org/stable/c/037e48ceccf163899374b601afb6ae8d0bf1d2ac
• https://git.kernel.org/stable/c/0fb74c00d140a66128afc0003785dcc57e69d312
• https://git.kernel.org/stable/c/7a7650b3ac23e5fc8c990f00e94f787dc84e3175
• https://git.kernel.org/stable/c/9e985cbf2942a1bb8fcef9adc2a17d90fd7ca8ee