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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id 13sm2347728ejw.88.2020.05.20.09.33.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 May 2020 09:33:41 -0700 (PDT) From: Vitaly Kuznetsov To: Maxim Levitsky , kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Maxim Levitsky Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally In-Reply-To: <20200520160740.6144-3-mlevitsk@redhat.com> References: <20200520160740.6144-1-mlevitsk@redhat.com> <20200520160740.6144-3-mlevitsk@redhat.com> Date: Wed, 20 May 2020 18:33:40 +0200 Message-ID: <874ksatvkr.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Maxim Levitsky writes: > This msr is only available when the host supports WAITPKG feature. > > This breaks a nested guest, if the L1 hypervisor is set to ignore > unknown msrs, because the only other safety check that the > kernel does is that it attempts to read the msr and > rejects it if it gets an exception. > > Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL > > Signed-off-by: Maxim Levitsky > --- > arch/x86/kvm/x86.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index fe3a24fd6b263..9c507b32b1b77 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void) > if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >= > min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp)) > continue; > + break; > + case MSR_IA32_UMWAIT_CONTROL: > + if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG)) > + continue; I'm probably missing something but (if I understand correctly) the only effect of dropping MSR_IA32_UMWAIT_CONTROL from msrs_to_save would be that KVM userspace won't see it in e.g. KVM_GET_MSR_INDEX_LIST. But why is this causing an issue? I see both vmx_get_msr()/vmx_set_msr() have 'host_initiated' check: case MSR_IA32_UMWAIT_CONTROL: if (!msr_info->host_initiated && !vmx_has_waitpkg(vmx)) return 1; so KVM userspace should be able to read/write this MSR even when there's no hardware support for it. Or who's trying to read/write it? Also, kvm_cpu_cap_has() check is not equal to vmx_has_waitpkg() which checks secondary execution controls. > default: > break; > } -- Vitaly