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KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02

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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>
This commit is contained in:
Sean Christopherson 2022-01-25 22:05:27 +00:00 committed by Paolo Bonzini
parent 4cf3d3ebe8
commit d6e656cd26
1 changed files with 12 additions and 10 deletions
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22
arch/x86/kvm/vmx/nested.c
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@ -4851,18 +4851,20 @@ 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;
}