selinux: fix NULL dereference in policydb_destroy()

The conversion to kvmalloc() forgot to account for the possibility that
p->type_attr_map_array might be null in policydb_destroy().

Fix this by destroying its contents only if it is not NULL.

Also make sure ebitmap_init() is called on all entries before
policydb_destroy() can be called. Right now this is a no-op, because
both kvcalloc() and ebitmap_init() just zero out the whole struct, but
let's rather not rely on a specific implementation.

Reported-by: syzbot+a57b2aff60832666fc28@syzkaller.appspotmail.com
Fixes: acdf52d9 ("selinux: convert to kvmalloc")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/policydb.c

@@ -828,9 +828,11 @@ void policydb_destroy(struct policydb *p)
 	hashtab_map(p->range_tr, range_tr_destroy, NULL);
 	hashtab_destroy(p->range_tr);
 
-	for (i = 0; i < p->p_types.nprim; i++)
-		ebitmap_destroy(&p->type_attr_map_array[i]);
-	kvfree(p->type_attr_map_array);
+	if (p->type_attr_map_array) {
+		for (i = 0; i < p->p_types.nprim; i++)
+			ebitmap_destroy(&p->type_attr_map_array[i]);
+		kvfree(p->type_attr_map_array);
+	}
 
 	ebitmap_destroy(&p->filename_trans_ttypes);
 	ebitmap_destroy(&p->policycaps);
@@ -2496,10 +2498,13 @@ int policydb_read(struct policydb *p, void *fp)
 	if (!p->type_attr_map_array)
 		goto bad;
 
+	/* just in case ebitmap_init() becomes more than just a memset(0): */
+	for (i = 0; i < p->p_types.nprim; i++)
+		ebitmap_init(&p->type_attr_map_array[i]);
+
 	for (i = 0; i < p->p_types.nprim; i++) {
 		struct ebitmap *e = &p->type_attr_map_array[i];
 
-		ebitmap_init(e);
 		if (p->policyvers >= POLICYDB_VERSION_AVTAB) {
 			rc = ebitmap_read(e, fp);
 			if (rc)