KVM: limit the number of pages per memory slot

This patch limits the number of pages per memory slot to make
us free from extra care about type issues.
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

include/linux/kvm_host.h

@@ -105,6 +105,12 @@ struct kvm_vcpu {
 	struct kvm_vcpu_arch arch;
 };
 
+/*
+ * Some of the bitops functions do not support too long bitmaps.
+ * This number must be determined not to exceed such limits.
+ */
+#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)
+
 struct kvm_memory_slot {
 	gfn_t base_gfn;
 	unsigned long npages;

virt/kvm/kvm_main.c

@@ -557,6 +557,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
 	npages = mem->memory_size >> PAGE_SHIFT;
 
+	r = -EINVAL;
+	if (npages > KVM_MEM_MAX_NR_PAGES)
+		goto out;
+
 	if (!npages)
 		mem->flags &= ~KVM_MEM_LOG_DIRTY_PAGES;
 
@@ -1187,13 +1191,10 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
 	memslot = gfn_to_memslot_unaliased(kvm, gfn);
 	if (memslot && memslot->dirty_bitmap) {
 		unsigned long rel_gfn = gfn - memslot->base_gfn;
-		unsigned long *p = memslot->dirty_bitmap +
-					rel_gfn / BITS_PER_LONG;
-		int offset = rel_gfn % BITS_PER_LONG;
 
 		/* avoid RMW */
-		if (!generic_test_le_bit(offset, p))
-			generic___set_le_bit(offset, p);
+		if (!generic_test_le_bit(rel_gfn, memslot->dirty_bitmap))
+			generic___set_le_bit(rel_gfn, memslot->dirty_bitmap);
 	}
 }