kvm: whitelist struct kvm_vcpu_arch

On x86, ARM and s390, struct kvm_vcpu_arch has a usercopy region
that is read and written by the KVM_GET/SET_CPUID2 ioctls (x86)
or KVM_GET/SET_ONE_REG (ARM/s390).  Without whitelisting the area,
KVM is completely broken on those architectures with usercopy hardening
enabled.

For now, allow writing to the entire struct on all architectures.
The KVM tree will not refine this to an architecture-specific
subset of struct kvm_vcpu_arch.

Cc: [email protected]
Cc: Kees Cook <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christoffer Dall <[email protected]>
Cc: Radim Krčmář <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Acked-by: Christoffer Dall <[email protected]>
Acked-by: Marc Zyngier <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

Author: bonzini

virt/kvm/kvm_main.c

@@ -4029,8 +4029,12 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
 	/* A kmem cache lets us meet the alignment requirements of fx_save. */
 	if (!vcpu_align)
 		vcpu_align = __alignof__(struct kvm_vcpu);
-	kvm_vcpu_cache = kmem_cache_create("kvm_vcpu", vcpu_size, vcpu_align,
-					   SLAB_ACCOUNT, NULL);
+	kvm_vcpu_cache =
+		kmem_cache_create_usercopy("kvm_vcpu", vcpu_size, vcpu_align,
+					   SLAB_ACCOUNT,
+					   offsetof(struct kvm_vcpu, arch),
+					   sizeof_field(struct kvm_vcpu, arch),
+					   NULL);
 	if (!kvm_vcpu_cache) {
 		r = -ENOMEM;
 		goto out_free_3;