KVM: nSVM: Check instead of asserting on nested TSC scaling support

commit 7cafe9b8e22bb3d77f130c461aedf6868c4aaf58 upstream.

Check for nested TSC scaling support on nested SVM VMRUN instead of
asserting that TSC scaling is exposed to L1 if L1's MSR_AMD64_TSC_RATIO
has diverged from KVM's default.  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: 164 PID: 62565 at arch/x86/kvm/svm/nested.c:699
           nested_vmcb02_prepare_control+0x3d6/0x3f0 [kvm_amd]
  Call Trace:
   <TASK>
   enter_svm_guest_mode+0x114/0x560 [kvm_amd]
   nested_svm_vmrun+0x260/0x330 [kvm_amd]
   vmrun_interception+0x29/0x30 [kvm_amd]
   svm_invoke_exit_handler+0x35/0x100 [kvm_amd]
   svm_handle_exit+0xe7/0x180 [kvm_amd]
   kvm_arch_vcpu_ioctl_run+0x1eab/0x2570 [kvm]
   kvm_vcpu_ioctl+0x4c9/0x5b0 [kvm]
   __se_sys_ioctl+0x7a/0xc0
   __x64_sys_ioctl+0x21/0x30
   do_syscall_64+0x41/0x90
   entry_SYSCALL_64_after_hwframe+0x63/0xcd
  RIP: 0033:0x45ca1b

Note, the nested #VMEXIT path has the same flaw, but needs a different
fix and will be handled separately.

Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230729011608.1065019-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 92268645a5fed0fdac78c211e1629f82be3d2d0a..798845699a58aad4a9c4591da52f9a06303c39fd 100644 (file)
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -660,10 +660,9 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 
        vmcb02->control.tsc_offset = vcpu->arch.tsc_offset;
 
-       if (svm->tsc_ratio_msr != kvm_caps.default_tsc_scaling_ratio) {
-               WARN_ON(!svm->tsc_scaling_enabled);
+       if (svm->tsc_scaling_enabled &&
+           svm->tsc_ratio_msr != kvm_caps.default_tsc_scaling_ratio)
                nested_svm_update_tsc_ratio_msr(vcpu);
-       }
 
        vmcb02->control.int_ctl             =
                (svm->nested.ctl.int_ctl & int_ctl_vmcb12_bits) |