KVM: x86: Split kvm_is_valid_cr4() and export only the non-vendor bits

Split the common x86 parts of kvm_is_valid_cr4(), i.e. the reserved bits
checks, into a separate helper, __kvm_is_valid_cr4(), and export only the
inner helper to vendor code in order to prevent nested VMX from calling
back into vmx_is_valid_cr4() via kvm_is_valid_cr4().

On SVM, this is a nop as SVM doesn't place any additional restrictions on
CR4.

On VMX, this is also currently a nop, but only because nested VMX is
missing checks on reserved CR4 bits for nested VM-Enter.  That bug will
be fixed in a future patch, and could simply use kvm_is_valid_cr4() as-is,
but nVMX has _another_ bug where VMXON emulation doesn't enforce VMX's
restrictions on CR0/CR4.  The cleanest and most intuitive way to fix the
VMXON bug is to use nested_host_cr{0,4}_valid().  If the CR4 variant
routes through kvm_is_valid_cr4(), using nested_host_cr4_valid() won't do
the right thing for the VMXON case as vmx_is_valid_cr4() enforces VMX's
restrictions if and only if the vCPU is post-VMXON.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220607213604.3346000-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b0cc911a8f6f..75ee509f0b7b 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3238,8 +3238,8 @@ static bool vmx_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
 	/*
 	 * We operate under the default treatment of SMM, so VMX cannot be
-	 * enabled under SMM.  Note, whether or not VMXE is allowed at all is
-	 * handled by kvm_is_valid_cr4().
+	 * enabled under SMM.  Note, whether or not VMXE is allowed at all,
+	 * i.e. is a reserved bit, is handled by common x86 code.
 	 */
 	if ((cr4 & X86_CR4_VMXE) && is_smm(vcpu))
 		return false;