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

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 an endless supply of:

------------[ cut here ]------------

WARNING: CPU: 10 PID: 206939 at arch/x86/kvm/svm/nested.c:1105

nested_svm_vmexit+0x6af/0x720 [kvm_amd]

Call Trace:

nested_svm_exit_handled+0x102/0x1f0 [kvm_amd]

svm_handle_exit+0xb9/0x180 [kvm_amd]

kvm_arch_vcpu_ioctl_run+0x1eab/0x2570 [kvm]

kvm_vcpu_ioctl+0x4c9/0x5b0 [kvm]

? trace_hardirqs_off+0x4d/0xa0

__se_sys_ioctl+0x7a/0xc0

__x64_sys_ioctl+0x21/0x30

do_syscall_64+0x41/0x90

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Unlike the nested VMRUN path, hoisting the svm->tsc_scaling_enabled check

into the if-statement is wrong as KVM needs to ensure L1's multiplier is

loaded in the above scenario. Alternatively, the WARN_ON() could simply

be deleted, but that would make KVM's behavior even more subtle, e.g. it's

not immediately obvious why it's safe to write MSR_AMD64_TSC_RATIO when

checking only tsc_ratio_msr.