Re: [PATCH MANUALSEL 5.16 4/8] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02

Paolo Bonzini
2022-02-10T16:36:59Z

On 2/9/22 19:56, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
>
> [ 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 <seanjc@google.com>
> Message-Id: <20220125220527.2093146-1-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> 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 c605c2c01394b..9cd68e1fcf602 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4827,18 +4827,20 @@ 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, it's just extra care but not particularly useful.

Paolo