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([2001:b07:6468:f312:63a7:c72e:ea0e:6045]) by smtp.googlemail.com with ESMTPSA id w12sm6590943ejv.43.2022.02.10.08.41.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Feb 2022 08:41:23 -0800 (PST) Message-ID: Date: Thu, 10 Feb 2022 17:41:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH MANUALSEL 5.15 4/8] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02 Content-Language: en-US To: Sasha Levin , linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Sean Christopherson , tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, kvm@vger.kernel.org References: <20220209185653.48833-1-sashal@kernel.org> <20220209185653.48833-4-sashal@kernel.org> From: Paolo Bonzini In-Reply-To: <20220209185653.48833-4-sashal@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/9/22 19:56, Sasha Levin wrote: > From: Sean Christopherson > > [ Upstream commit d6e656cd266cdcc95abd372c7faef05bee271d1a ] > > WARN if KVM attempts to allocate a shadow VMCS for vmcs02. KVM emulates > VMCS shadowing but doesn't virtualize it, i.e. KVM should never allocate > a "real" shadow VMCS for L2. > > The previous code WARNed but continued anyway with the allocation, > presumably in an attempt to avoid NULL pointer dereference. > However, alloc_vmcs (and hence alloc_shadow_vmcs) can fail, and > indeed the sole caller does: > > if (enable_shadow_vmcs && !alloc_shadow_vmcs(vcpu)) > goto out_shadow_vmcs; > > which makes it not a useful attempt. > > Signed-off-by: Sean Christopherson > Message-Id: <20220125220527.2093146-1-seanjc@google.com> > Signed-off-by: Paolo Bonzini > Signed-off-by: Sasha Levin > --- > arch/x86/kvm/vmx/nested.c | 22 ++++++++++++---------- > 1 file changed, 12 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index a0193b11c381d..42559c21fb2cc 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -4840,18 +4840,20 @@ static struct vmcs *alloc_shadow_vmcs(struct kvm_vcpu *vcpu) > struct loaded_vmcs *loaded_vmcs = vmx->loaded_vmcs; > > /* > - * We should allocate a shadow vmcs for vmcs01 only when L1 > - * executes VMXON and free it when L1 executes VMXOFF. > - * As it is invalid to execute VMXON twice, we shouldn't reach > - * here when vmcs01 already have an allocated shadow vmcs. > + * KVM allocates a shadow VMCS only when L1 executes VMXON and frees it > + * when L1 executes VMXOFF or the vCPU is forced out of nested > + * operation. VMXON faults if the CPU is already post-VMXON, so it > + * should be impossible to already have an allocated shadow VMCS. KVM > + * doesn't support virtualization of VMCS shadowing, so vmcs01 should > + * always be the loaded VMCS. > */ > - WARN_ON(loaded_vmcs == &vmx->vmcs01 && loaded_vmcs->shadow_vmcs); > + if (WARN_ON(loaded_vmcs != &vmx->vmcs01 || loaded_vmcs->shadow_vmcs)) > + return loaded_vmcs->shadow_vmcs; > + > + loaded_vmcs->shadow_vmcs = alloc_vmcs(true); > + if (loaded_vmcs->shadow_vmcs) > + vmcs_clear(loaded_vmcs->shadow_vmcs); > > - if (!loaded_vmcs->shadow_vmcs) { > - loaded_vmcs->shadow_vmcs = alloc_vmcs(true); > - if (loaded_vmcs->shadow_vmcs) > - vmcs_clear(loaded_vmcs->shadow_vmcs); > - } > return loaded_vmcs->shadow_vmcs; > } > NACK Paolo