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 KVM's 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 doesn't 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 isn't 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() doesn't clear the upper

bits either, i.e. leaves ICL_FIXED_0_ADAPTIVE set, and

intel_pmu_enable_fixed() effectively doesn't 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 doesn't 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, wouldn't be guaranteed to

work (i.e. KVM might still kill/confuse the guest), and it's 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 doesn't

support PEBS in any capacity.