net: shaper: protect from late creation of hierarchy

We look up a netdev during prep of Netlink ops (pre- callbacks)
and take a ref to it. Then later in the body of the callback
we take its lock or RCU which are the actual protections.

The netdev may get unregistered in between the time we take
the ref and the time we lock it. We may allocate the hierarchy
after flush has already run, which would lead to a leak.

Take the instance lock in pre- already, this saves us from the race
and removes the need for dedicated lock/unlock callbacks completely.
After all, if there's any chance of write happening concurrently
with the flush - we're back to leaking the hierarchy.

We may take the lock for devices which don't support shapers but
we're only dealing with SET operations here, not taking the lock
would be optimizing for an error case.

Fixes: 93954b40f6a4 ("net-shapers: implement NL set and delete operations")
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

Documentation/netlink/specs/net_shaper.yaml

···
       flags: [admin-perm]
 
       do:
-        pre: net-shaper-nl-pre-doit
-        post: net-shaper-nl-post-doit
+        pre: net-shaper-nl-pre-doit-write
+        post: net-shaper-nl-post-doit-write
         request:
           attributes:
             - ifindex
···
       flags: [admin-perm]
 
       do:
-        pre: net-shaper-nl-pre-doit
-        post: net-shaper-nl-post-doit
+        pre: net-shaper-nl-pre-doit-write
+        post: net-shaper-nl-post-doit-write
         request:
           attributes: *ns-binding
 
···
       flags: [admin-perm]
 
       do:
-        pre: net-shaper-nl-pre-doit
-        post: net-shaper-nl-post-doit
+        pre: net-shaper-nl-pre-doit-write
+        post: net-shaper-nl-post-doit-write
         request:
           attributes:
             - ifindex

net/shaper/shaper.c

···
 	return &((struct net_shaper_nl_ctx *)ctx)->binding;
 }
 
-static void net_shaper_lock(struct net_shaper_binding *binding)
-{
-	switch (binding->type) {
-	case NET_SHAPER_BINDING_TYPE_NETDEV:
-		netdev_lock(binding->netdev);
-		break;
-	}
-}
-
-static void net_shaper_unlock(struct net_shaper_binding *binding)
-{
-	switch (binding->type) {
-	case NET_SHAPER_BINDING_TYPE_NETDEV:
-		netdev_unlock(binding->netdev);
-		break;
-	}
-}
-
 static struct net_shaper_hierarchy *
 net_shaper_hierarchy(struct net_shaper_binding *binding)
 {
···
 	return 0;
 }
 
+/* Like net_shaper_ctx_setup(), but for "write" handlers (never for dumps!)
+ * Acquires the lock protecting the hierarchy (instance lock for netdev).
+ */
+static int net_shaper_ctx_setup_lock(const struct genl_info *info, int type,
+				     struct net_shaper_nl_ctx *ctx)
+{
+	struct net *ns = genl_info_net(info);
+	struct net_device *dev;
+	int ifindex;
+
+	if (GENL_REQ_ATTR_CHECK(info, type))
+		return -EINVAL;
+
+	ifindex = nla_get_u32(info->attrs[type]);
+	dev = netdev_get_by_index_lock(ns, ifindex);
+	if (!dev) {
+		NL_SET_BAD_ATTR(info->extack, info->attrs[type]);
+		return -ENOENT;
+	}
+
+	if (!dev->netdev_ops->net_shaper_ops) {
+		NL_SET_BAD_ATTR(info->extack, info->attrs[type]);
+		netdev_unlock(dev);
+		return -EOPNOTSUPP;
+	}
+
+	ctx->binding.type = NET_SHAPER_BINDING_TYPE_NETDEV;
+	ctx->binding.netdev = dev;
+	return 0;
+}
+
 static void net_shaper_ctx_cleanup(struct net_shaper_nl_ctx *ctx)
 {
 	if (ctx->binding.type == NET_SHAPER_BINDING_TYPE_NETDEV)
 		netdev_put(ctx->binding.netdev, &ctx->dev_tracker);
+}
+
+static void net_shaper_ctx_cleanup_unlock(struct net_shaper_nl_ctx *ctx)
+{
+	if (ctx->binding.type == NET_SHAPER_BINDING_TYPE_NETDEV)
+		netdev_unlock(ctx->binding.netdev);
 }
 
 static u32 net_shaper_handle_to_index(const struct net_shaper_handle *handle)
···
 }
 
 /* Allocate on demand the per device shaper's hierarchy container.
- * Called under the net shaper lock
+ * Called under the lock protecting the hierarchy (instance lock for netdev)
  */
 static struct net_shaper_hierarchy *
 net_shaper_hierarchy_setup(struct net_shaper_binding *binding)
···
 	net_shaper_generic_post(info);
 }
 
+int net_shaper_nl_pre_doit_write(const struct genl_split_ops *ops,
+				struct sk_buff *skb, struct genl_info *info)
+{
+	struct net_shaper_nl_ctx *ctx = (struct net_shaper_nl_ctx *)info->ctx;
+
+	BUILD_BUG_ON(sizeof(*ctx) > sizeof(info->ctx));
+
+	return net_shaper_ctx_setup_lock(info, NET_SHAPER_A_IFINDEX, ctx);
+}
+
+void net_shaper_nl_post_doit_write(const struct genl_split_ops *ops,
+				   struct sk_buff *skb, struct genl_info *info)
+{
+	net_shaper_ctx_cleanup_unlock((struct net_shaper_nl_ctx *)info->ctx);
+}
+
 int net_shaper_nl_pre_dumpit(struct netlink_callback *cb)
 {
 	struct net_shaper_nl_ctx *ctx = (struct net_shaper_nl_ctx *)cb->ctx;
···
 
 	binding = net_shaper_binding_from_ctx(info->ctx);
 
-	net_shaper_lock(binding);
 	ret = net_shaper_parse_info(binding, info->attrs, info, &shaper,
 				    &exists);
 	if (ret)
-		goto unlock;
+		return ret;
 
 	if (!exists)
 		net_shaper_default_parent(&shaper.handle, &shaper.parent);
 
 	hierarchy = net_shaper_hierarchy_setup(binding);
-	if (!hierarchy) {
-		ret = -ENOMEM;
-		goto unlock;
-	}
+	if (!hierarchy)
+		return -ENOMEM;
 
 	/* The 'set' operation can't create node-scope shapers. */
 	handle = shaper.handle;
 	if (handle.scope == NET_SHAPER_SCOPE_NODE &&
-	    !net_shaper_lookup(binding, &handle)) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	    !net_shaper_lookup(binding, &handle))
+		return -ENOENT;
 
 	ret = net_shaper_pre_insert(binding, &handle, info->extack);
 	if (ret)
-		goto unlock;
+		return ret;
 
 	ops = net_shaper_ops(binding);
 	ret = ops->set(binding, &shaper, info->extack);
 	if (ret) {
 		net_shaper_rollback(binding);
-		goto unlock;
+		return ret;
 	}
 
 	net_shaper_commit(binding, 1, &shaper);
 
-unlock:
-	net_shaper_unlock(binding);
-	return ret;
+	return 0;
 }
 
 static int __net_shaper_delete(struct net_shaper_binding *binding,
···
 
 	binding = net_shaper_binding_from_ctx(info->ctx);
 
-	net_shaper_lock(binding);
 	ret = net_shaper_parse_handle(info->attrs[NET_SHAPER_A_HANDLE], info,
 				      &handle);
 	if (ret)
-		goto unlock;
+		return ret;
 
 	hierarchy = net_shaper_hierarchy(binding);
-	if (!hierarchy) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	if (!hierarchy)
+		return -ENOENT;
 
 	shaper = net_shaper_lookup(binding, &handle);
-	if (!shaper) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	if (!shaper)
+		return -ENOENT;
 
 	if (handle.scope == NET_SHAPER_SCOPE_NODE) {
 		ret = net_shaper_pre_del_node(binding, shaper, info->extack);
 		if (ret)
-			goto unlock;
+			return ret;
 	}
 
-	ret = __net_shaper_delete(binding, shaper, info->extack);
-
-unlock:
-	net_shaper_unlock(binding);
-	return ret;
+	return __net_shaper_delete(binding, shaper, info->extack);
 }
 
 static int net_shaper_group_send_reply(struct net_shaper_binding *binding,
···
 	if (!net_shaper_ops(binding)->group)
 		return -EOPNOTSUPP;
 
-	net_shaper_lock(binding);
 	leaves_count = net_shaper_list_len(info, NET_SHAPER_A_LEAVES);
 	if (!leaves_count) {
 		NL_SET_BAD_ATTR(info->extack,
 				info->attrs[NET_SHAPER_A_LEAVES]);
-		ret = -EINVAL;
-		goto unlock;
+		return -EINVAL;
 	}
 
 	leaves = kcalloc(leaves_count, sizeof(struct net_shaper) +
 			 sizeof(struct net_shaper *), GFP_KERNEL);
-	if (!leaves) {
-		ret = -ENOMEM;
-		goto unlock;
-	}
+	if (!leaves)
+		return -ENOMEM;
 	old_nodes = (void *)&leaves[leaves_count];
 
 	ret = net_shaper_parse_node(binding, info->attrs, info, &node);
···
 
 free_leaves:
 	kfree(leaves);
-
-unlock:
-	net_shaper_unlock(binding);
 	return ret;
 
 free_msg:
···
 	if (!hierarchy)
 		return;
 
-	net_shaper_lock(binding);
 	xa_lock(&hierarchy->shapers);
 	xa_for_each(&hierarchy->shapers, index, cur) {
 		__xa_erase(&hierarchy->shapers, index);
 		kfree(cur);
 	}
 	xa_unlock(&hierarchy->shapers);
-	net_shaper_unlock(binding);
 
 	kfree(hierarchy);
 }

net/shaper/shaper_nl_gen.c

···
 	},
 	{
 		.cmd		= NET_SHAPER_CMD_SET,
-		.pre_doit	= net_shaper_nl_pre_doit,
+		.pre_doit	= net_shaper_nl_pre_doit_write,
 		.doit		= net_shaper_nl_set_doit,
-		.post_doit	= net_shaper_nl_post_doit,
+		.post_doit	= net_shaper_nl_post_doit_write,
 		.policy		= net_shaper_set_nl_policy,
 		.maxattr	= NET_SHAPER_A_IFINDEX,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 	{
 		.cmd		= NET_SHAPER_CMD_DELETE,
-		.pre_doit	= net_shaper_nl_pre_doit,
+		.pre_doit	= net_shaper_nl_pre_doit_write,
 		.doit		= net_shaper_nl_delete_doit,
-		.post_doit	= net_shaper_nl_post_doit,
+		.post_doit	= net_shaper_nl_post_doit_write,
 		.policy		= net_shaper_delete_nl_policy,
 		.maxattr	= NET_SHAPER_A_IFINDEX,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 	{
 		.cmd		= NET_SHAPER_CMD_GROUP,
-		.pre_doit	= net_shaper_nl_pre_doit,
+		.pre_doit	= net_shaper_nl_pre_doit_write,
 		.doit		= net_shaper_nl_group_doit,
-		.post_doit	= net_shaper_nl_post_doit,
+		.post_doit	= net_shaper_nl_post_doit_write,
 		.policy		= net_shaper_group_nl_policy,
 		.maxattr	= NET_SHAPER_A_LEAVES,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,

net/shaper/shaper_nl_gen.h

···
 
 int net_shaper_nl_pre_doit(const struct genl_split_ops *ops,
 			   struct sk_buff *skb, struct genl_info *info);
+int net_shaper_nl_pre_doit_write(const struct genl_split_ops *ops,
+				 struct sk_buff *skb, struct genl_info *info);
 int net_shaper_nl_cap_pre_doit(const struct genl_split_ops *ops,
 			       struct sk_buff *skb, struct genl_info *info);
 void
 net_shaper_nl_post_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
 			struct genl_info *info);
+void
+net_shaper_nl_post_doit_write(const struct genl_split_ops *ops,
+			      struct sk_buff *skb, struct genl_info *info);
 void
 net_shaper_nl_cap_post_doit(const struct genl_split_ops *ops,
 			    struct sk_buff *skb, struct genl_info *info);