[PATCH] check_netdev_priv: warn about using netdev priv data after free_netdev

[Pavel Skripkin <paskripkin@gmail.com>]

After manual code review I found a lot of bugs, when code uses
netdev_priv() pointer after free_{netdev,candev} call. Example:

	struct some_dev *dev = netdev_priv(net_dev);

	free_netdev(net_dev);
	do_clean_up(dev);

Obviously, that above code snippet will trigger UAF bug, since dev
pointer goes away with net_dev. Since there isn't any check for this
type of bug in other static analysis tools, let's add it to smatch.

Side note: this code requires further work to be able to find complex
situtations like

	struct some_dev *dev = get_dev_from_smth(smth);

	free_netdev(dev->netdev);
	do_clean_up(dev);

In this case dev is dev->netdev private data and it's a bit difficult to
to figure out if ->netdev is actual dev _parent_ or not.

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 check_list.h        |   1 +
 check_netdev_priv.c | 117 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 118 insertions(+)
 create mode 100644 check_netdev_priv.c

diff --git a/check_list.h b/check_list.h
index fd205269..972b2795 100644
--- a/check_list.h
+++ b/check_list.h
@@ -203,6 +203,7 @@ CK(check_list_add)
 CK(check_list_add_late)
 CK(check_sscanf_return)
 CK(check_kvmalloc_NOFS)
+CK(check_uaf_netdev_priv)
 
 /* wine specific stuff */
 CK(check_wine_filehandles)
diff --git a/check_netdev_priv.c b/check_netdev_priv.c
new file mode 100644
index 00000000..87d373b5
--- /dev/null
+++ b/check_netdev_priv.c
@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2021 Pavel Skripkin
+ */
+
+/* TODO:
+ *
+ * Try to find a way how to handle situations like:
+ *
+ *	struct some_dev *dev = get_dev_from_smth(smth);
+ *
+ *	free_netdev(dev->netdev);
+ *	do_clean_up(dev);
+ *
+ *
+ *	In this case dev is dev->netdev private data (exmpl: ems_usb_disconnect())
+ */
+
+#include "smatch.h"
+#include "smatch_extra.h"
+#include "smatch_function_hashtable.h"
+
+static int my_id;
+STATE(freed);
+STATE(ok);
+
+static void ok_to_use(struct sm_state *sm, struct expression *mod_expr)
+{
+	if (sm->state != &ok)
+		set_state(my_id, sm->name, sm->sym, &ok);
+}
+
+static inline char *get_function_name(struct expression *expr)
+{
+	if (!expr || expr->type != EXPR_CALL)
+		return NULL;
+	if (expr->fn->type != EXPR_SYMBOL || !expr->fn->symbol_name)
+		return NULL;
+	return expr->fn->symbol_name->name;
+}
+
+static inline int is_netdev_priv(struct expression *call)
+{
+	char *name;
+
+	if (!call || call->type != EXPR_CALL)
+		return 0;
+
+	name = get_function_name(call);
+	if (!name)
+		return 0;
+
+	return !strcmp("netdev_priv", name);
+}
+
+static const char *get_parent_netdev_name(struct expression *expr)
+{
+	struct expression *call, *arg_expr = NULL;
+	struct symbol *sym;
+
+	call = get_assigned_expr(strip_expr(expr));
+	if (is_netdev_priv(call)) {
+		arg_expr = get_argument_from_call_expr(call->args, 0);
+		arg_expr = strip_expr(arg_expr);
+	} else {
+		return NULL;
+	}
+
+	return expr_to_var_sym(arg_expr, &sym);
+}
+
+static void match_free_netdev(const char *fn, struct expression *expr, void *_arg_no)
+{
+	struct expression *arg;
+	const char *name;
+
+	arg = get_argument_from_call_expr(expr->args, PTR_INT(_arg_no));
+	if (!arg)
+		return;
+
+	name = expr_to_var(arg);
+	if (!name)
+		return;
+
+	set_state(my_id, name, NULL, &freed);
+}
+
+static void match_symbol(struct expression *expr)
+{
+	const char *parent_netdev, *name;
+	struct smatch_state *state;
+
+	name = expr_to_var(expr);
+	if (!name)
+		return;
+
+	parent_netdev = get_parent_netdev_name(expr);
+	if (!parent_netdev)
+		return;
+	
+	state = get_state(my_id, parent_netdev, NULL);
+	if (state == &freed)
+		sm_error("Using %s after free_{netdev,candev}(%s);\n", name, parent_netdev);
+}
+
+void check_uaf_netdev_priv(int id)
+{
+	if (option_project != PROJ_KERNEL)
+		return;
+
+	my_id = id;
+
+	add_function_hook("free_netdev", &match_free_netdev, NULL);
+	add_function_hook("free_candev", &match_free_netdev, NULL);
+	add_modification_hook(my_id, &ok_to_use);
+	add_hook(&match_symbol, SYM_HOOK);
+}
-- 
2.32.0