CVE-2023-53208

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

KVM: nSVM: Load L1s TSC multiplier based on L1 state, not L2 state

When emulating nested VM-Exit, load L1s TSC multiplier if L1s desired ratio doesnt 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 L1s multiplier if L2s multiplier is the default, i.e. checking that L1s 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 KVMs 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 L1s multiplier is loaded in the above scenario. Alternatively, the WARN_ON() could simply be deleted, but that would make KVMs behavior even more subtle, e.g. its not immediately obvious why its safe to write MSR_AMD64_TSC_RATIO when checking only tsc_ratio_msr.

Affected Software

References

• https://git.kernel.org/stable/c/0c94e2468491cbf0754f49a5136ab51294a96b69
• https://git.kernel.org/stable/c/5b2b0535fa7adee7e295fed0a3095082131a8d05
• https://git.kernel.org/stable/c/e91c07f6cf7060d2acb3aeee31a6baebe3773d3f