drm/i915/gvt/kvmgt: check returned slot for gfn

gfn_to_memslot() may return NULL if the gfn is mmio
or invalid. A malicious user might input a bad gfn
to panic the host if we don't check it.
Signed-off-by: NJike Song <jike.song@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -1137,6 +1137,10 @@ static int kvmgt_write_protect_add(unsigned long handle, u64 gfn)
 
 	idx = srcu_read_lock(&kvm->srcu);
 	slot = gfn_to_memslot(kvm, gfn);
+	if (!slot) {
+		srcu_read_unlock(&kvm->srcu, idx);
+		return -EINVAL;
+	}
 
 	spin_lock(&kvm->mmu_lock);
 
@@ -1167,6 +1171,10 @@ static int kvmgt_write_protect_remove(unsigned long handle, u64 gfn)
 
 	idx = srcu_read_lock(&kvm->srcu);
 	slot = gfn_to_memslot(kvm, gfn);
+	if (!slot) {
+		srcu_read_unlock(&kvm->srcu, idx);
+		return -EINVAL;
+	}
 
 	spin_lock(&kvm->mmu_lock);