KVM: MMU: Segregate shadow pages with different cr0.wp

When cr0.wp=0, we may shadow a gpte having u/s=1 and r/w=0 with an spte
having u/s=0 and r/w=1.  This allows excessive access if the guest sets
cr0.wp=1 and accesses through this spte.

Fix by making cr0.wp part of the base role; we'll have different sptes for
the two cases and the problem disappears.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Documentation/kvm/mmu.txt

@@ -163,6 +163,8 @@ Shadow pages contain the following information:
     32-bit or 64-bit gptes are in use).
   role.cr4_nxe:
     Contains the value of efer.nxe for which the page is valid.
+  role.cr0_wp:
+    Contains the value of cr0.wp for which the page is valid.
   gfn:
     Either the guest page table containing the translations shadowed by this
     page, or the base page frame for linear translations.  See role.direct.

arch/x86/include/asm/kvm_host.h

@@ -179,6 +179,7 @@ union kvm_mmu_page_role {
 		unsigned access:3;
 		unsigned invalid:1;
 		unsigned nxe:1;
+		unsigned cr0_wp:1;
 	};
 };
 

arch/x86/kvm/mmu.c

@@ -217,7 +217,7 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_set_mask_ptes);
 
-static int is_write_protection(struct kvm_vcpu *vcpu)
+static bool is_write_protection(struct kvm_vcpu *vcpu)
 {
 	return kvm_read_cr0_bits(vcpu, X86_CR0_WP);
 }
@@ -2432,6 +2432,7 @@ static int init_kvm_softmmu(struct kvm_vcpu *vcpu)
 		r = paging32_init_context(vcpu);
 
 	vcpu->arch.mmu.base_role.cr4_pae = !!is_pae(vcpu);
+	vcpu->arch.mmu.base_role.cr0_wp = is_write_protection(vcpu);
 
 	return r;
 }