module: verify_export_symbols under the lock

It disabled preempt so it was "safe", but nothing stops another module
slipping in before this module is added to the global list now we don't
hold the lock the whole time.

So we check this just after we check for duplicate modules, and just
before we put the module in the global list.

(find_symbol finds symbols in coming and going modules, too).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/module.c

@@ -1571,6 +1571,8 @@ EXPORT_SYMBOL_GPL(__symbol_get);
 /*
  * Ensure that an exported symbol [global namespace] does not already exist
  * in the kernel or in some other module's exported symbol table.
+ *
+ * You must hold the module_mutex.
  */
 static int verify_export_symbols(struct module *mod)
 {
@@ -1592,14 +1594,7 @@ static int verify_export_symbols(struct module *mod)
 
 	for (i = 0; i < ARRAY_SIZE(arr); i++) {
 		for (s = arr[i].sym; s < arr[i].sym + arr[i].num; s++) {
-			const struct kernel_symbol *sym;
-
-			/* Stopping preemption makes find_symbol safe. */
-			preempt_disable();
-			sym = find_symbol(s->name, &owner, NULL, true, false);
-			preempt_enable();
-
-			if (sym) {
+			if (find_symbol(s->name, &owner, NULL, true, false)) {
 				printk(KERN_ERR
 				       "%s: exports duplicate symbol %s"
 				       " (owned by %s)\n",
@@ -2440,11 +2435,6 @@ static noinline struct module *load_module(void __user *umod,
 			goto cleanup;
 	}
 
-        /* Find duplicate symbols */
-	err = verify_export_symbols(mod);
-	if (err < 0)
-		goto cleanup;
-
   	/* Set up and sort exception table */
 	mod->extable = section_objs(hdr, sechdrs, secstrings, "__ex_table",
 				    sizeof(*mod->extable), &mod->num_exentries);
@@ -2506,10 +2496,14 @@ static noinline struct module *load_module(void __user *umod,
 	mutex_lock(&module_mutex);
 	if (find_module(mod->name)) {
 		err = -EEXIST;
-		/* This will also unlock the mutex */
-		goto already_exists;
+		goto unlock;
 	}
 
+	/* Find duplicate symbols */
+	err = verify_export_symbols(mod);
+	if (err < 0)
+		goto unlock;
+
 	list_add_rcu(&mod->list, &modules);
 	mutex_unlock(&module_mutex);
 
@@ -2536,7 +2530,7 @@ static noinline struct module *load_module(void __user *umod,
 	mutex_lock(&module_mutex);
 	/* Unlink carefully: kallsyms could be walking list. */
 	list_del_rcu(&mod->list);
- already_exists:
+ unlock:
 	mutex_unlock(&module_mutex);
 	synchronize_sched();
 	module_arch_cleanup(mod);