CVE-2023-53208

/// Technical description

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

KVM: nSVM: Load L1's TSC multiplier based on L1 state, not L2 state

When emulating nested VM-Exit, load L1's TSC multiplier if L1's desired

ratio doesn't match the current ratio, not if the ratio L1 is using for

L2 diverges from the default. Functionally, the end result is the same

as KVM will run L2 with L1's multiplier if L2's multiplier is the default,

i.e. checking that L1's multiplier is loaded is equivalent to checking if

L2 has a non-default multiplier.

However, the assertion that TSC scaling is exposed to L1 is flawed, as

userspace can trigger the WARN at will by writing the MSR and then

updating guest CPUID to hide the feature (modifying guest CPUID is

allowed anytime before KVM_RUN). E.g. hacking KVM's state_test

selftest to do

vcpu_set_msr(vcpu, MSR_AMD64_TSC_RATIO, 0);

vcpu_clear_cpuid_feature(vcpu, X86_FEATURE_TSCRATEMSR);

after restoring state in a new VM+vCPU yields

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References