libsepol: check decl_id bounds before using it

When loading an invalid module which uses a declaration ID 0,
semodule_package crashes in policydb_index_decls():

    p->decl_val_to_struct[decl->decl_id - 1] = decl;

gdb shows the following stack trace:

    #0  0x00007ffff7aa1bbd in policydb_index_decls (p=p@entry=0x605360)
    at policydb.c:1034
    #1  0x00007ffff7aaa9fc in policydb_read (p=<optimized out>,
    fp=fp@entry=0x605090, verbose=verbose@entry=0) at policydb.c:3958
    #2  0x00007ffff7ab4764 in sepol_policydb_read (p=<optimized out>,
    pf=pf@entry=0x605090) at policydb_public.c:174
    #3  0x0000000000401d33 in main (argc=<optimized out>,
    argv=0x7fffffffdc88) at semodule_package.c:220

Change policydb_index_decls() to report an error instead:

    libsepol.policydb_index_decls: invalid decl ID 0

Signed-off-by: Nicolas Iooss <[email protected]>

libsepol/src/policydb.c

@@ -1090,11 +1090,11 @@ int policydb_index_bools(policydb_t * p)
 	return 0;
 }
 
-int policydb_index_decls(policydb_t * p)
+int policydb_index_decls(sepol_handle_t * handle, policydb_t * p)
 {
 	avrule_block_t *curblock;
 	avrule_decl_t *decl;
-	int num_decls = 0;
+	unsigned int num_decls = 0;
 
 	free(p->decl_val_to_struct);
 
@@ -1114,6 +1114,10 @@ int policydb_index_decls(policydb_t * p)
 	for (curblock = p->global; curblock != NULL; curblock = curblock->next) {
 		for (decl = curblock->branch_list; decl != NULL;
 		     decl = decl->next) {
+			if (decl->decl_id < 1 || decl->decl_id > num_decls) {
+				ERR(handle, "invalid decl ID %u", decl->decl_id);
+				return -1;
+			}
 			p->decl_val_to_struct[decl->decl_id - 1] = decl;
 		}
 	}
@@ -4039,7 +4043,7 @@ int policydb_read(policydb_t * p, struct policy_file *fp, unsigned verbose)
 
 	}
 
-	if (policydb_index_decls(p))
+	if (policydb_index_decls(fp->handle, p))
 		goto bad;
 
 	if (policydb_index_classes(p))