KVM: x86: nSVM: test eax for 4K alignment for GP errata workaround

GP SVM errata workaround made the #GP handler always emulate
the SVM instructions.

However these instructions #GP in case the operand is not 4K aligned,
but the workaround code didn't check this and we ended up
emulating these instructions anyway.

This is only an emulation accuracy check bug as there is no harm for
KVM to read/write unaligned vmcb images.

Fixes: 82a11e9c ("KVM: SVM: Add emulation support for #GP triggered by SVM instructions")
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210914154825.104886-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -2224,6 +2224,10 @@ static int gp_interception(struct kvm_vcpu *vcpu)
 	if (error_code)
 		goto reinject;
 
+	/* All SVM instructions expect page aligned RAX */
+	if (svm->vmcb->save.rax & ~PAGE_MASK)
+		goto reinject;
+
 	/* Decode the instruction for usage later */
 	if (x86_decode_emulated_instruction(vcpu, 0, NULL, 0) != EMULATION_OK)
 		goto reinject;