Subject: Re: [RESEND PATCH] bonding: wait for sysfs kobject destruction before freeing struct slave

From: Jamie Iles <[email-masked]>
Date: Thu, 5 Nov 2020 18:11:08 +0000

Hi Eric,

On Thu, Nov 05, 2020 at 01:49:03PM +0100, Eric Dumazet wrote:
> On 11/5/20 9:41 AM, Jamie Iles wrote:
> > syzkaller found that with CONFIG_DEBUG_KOBJECT_RELEASE=y, releasing a
> > struct slave device could result in the following splat:
> > 
> >
> 
> > This is a potential use-after-free if the sysfs nodes are being accessed
> > whilst removing the struct slave, so wait for the object destruction to
> > complete before freeing the struct slave itself.
> > 
> > Fixes: 07699f9a7c8d ("bonding: add sysfs /slave dir for bond slave devices.")
> > Fixes: a068aab42258 ("bonding: Fix reference count leak in bond_sysfs_slave_add.")
> > Cc: Qiushi Wu <[email-masked]>
> > Cc: Jay Vosburgh <[email-masked]>
> > Cc: Veaceslav Falico <[email-masked]>
> > Cc: Andy Gospodarek <[email-masked]>
> > Signed-off-by: Jamie Iles <[email-masked]>
> > ---
...
> This seems weird, are we going to wait for a completion while RTNL is held ?
> I am pretty sure this could be exploited by malicious user/syzbot.
> 
> The .release() handler could instead perform a refcounted
> bond_free_slave() action.

Okay, so were you thinking along the lines of this moving the lifetime 
of the slave to the kobject?

Thanks,

Jamie

diff --git i/drivers/net/bonding/bond_main.c w/drivers/net/bonding/bond_main.c
index 84ecbc6fa0ff..ea8ecc6e87c2 100644
--- i/drivers/net/bonding/bond_main.c
+++ w/drivers/net/bonding/bond_main.c
@@ -1481,7 +1481,7 @@ static struct slave *bond_alloc_slave(struct bonding *bond)
 	return slave;
 }
 
-static void bond_free_slave(struct slave *slave)
+void bond_free_slave(struct slave *slave)
 {
 	struct bonding *bond = bond_get_bond_by_slave(slave);
 
@@ -1691,6 +1691,10 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	 */
 	new_slave->queue_id = 0;
 
+	res = bond_slave_kobj_init(new_slave);
+	if (res)
+		goto err_free;
+
 	/* Save slave's original mtu and then set it to match the bond */
 	new_slave->original_mtu = slave_dev->mtu;
 	res = dev_set_mtu(slave_dev, bond->dev->mtu);
@@ -1912,7 +1916,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 		if (bond_dev->flags & IFF_PROMISC) {
 			res = dev_set_promiscuity(slave_dev, 1);
 			if (res)
-				goto err_sysfs_del;
+				goto err_upper_unlink;
 		}
 
 		/* set allmulti level to new slave */
@@ -1921,7 +1925,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 			if (res) {
 				if (bond_dev->flags & IFF_PROMISC)
 					dev_set_promiscuity(slave_dev, -1);
-				goto err_sysfs_del;
+				goto err_upper_unlink;
 			}
 		}
 
@@ -1961,9 +1965,6 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	return 0;
 
 /* Undo stages on error */
-err_sysfs_del:
-	bond_sysfs_slave_del(new_slave);
-
 err_upper_unlink:
 	bond_upper_dev_unlink(bond, new_slave);
 
@@ -2007,7 +2008,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	dev_set_mtu(slave_dev, new_slave->original_mtu);
 
 err_free:
-	bond_free_slave(new_slave);
+	bond_slave_kobj_put(new_slave);
 
 err_undo_flags:
 	/* Enslave of first slave has failed and we need to fix master's mac */
@@ -2066,8 +2067,6 @@ static int __bond_release_one(struct net_device *bond_dev,
 
 	bond_set_slave_inactive_flags(slave, BOND_SLAVE_NOTIFY_NOW);
 
-	bond_sysfs_slave_del(slave);
-
 	/* recompute stats just before removing the slave */
 	bond_get_stats(bond->dev, &bond->bond_stats);
 
@@ -2187,7 +2186,7 @@ static int __bond_release_one(struct net_device *bond_dev,
 	if (!netif_is_bond_master(slave_dev))
 		slave_dev->priv_flags &= ~IFF_BONDING;
 
-	bond_free_slave(slave);
+	bond_slave_kobj_put(slave);
 
 	return 0;
 }
diff --git i/drivers/net/bonding/bond_sysfs_slave.c w/drivers/net/bonding/bond_sysfs_slave.c
index 9b8346638f69..67732078ef26 100644
--- i/drivers/net/bonding/bond_sysfs_slave.c
+++ w/drivers/net/bonding/bond_sysfs_slave.c
@@ -136,24 +136,35 @@ static const struct sysfs_ops slave_sysfs_ops = {
 	.show = slave_show,
 };
 
+static void slave_release(struct kobject *kobj)
+{
+	struct slave *slave = to_slave(kobj);
+
+	bond_free_slave(slave);
+}
+
 static struct kobj_type slave_ktype = {
+	.release = slave_release,
 #ifdef CONFIG_SYSFS
 	.sysfs_ops = &slave_sysfs_ops,
 #endif
 };
 
+int bond_slave_kobj_init(struct slave *slave)
+{
+	int err = kobject_init_and_add(&slave->kobj, &slave_ktype,
+				       &(slave->dev->dev.kobj), "bonding_slave");
+	if (err)
+		kobject_put(&slave->kobj);
+
+	return err;
+}
+
 int bond_sysfs_slave_add(struct slave *slave)
 {
 	const struct slave_attribute **a;
 	int err;
 
-	err = kobject_init_and_add(&slave->kobj, &slave_ktype,
-				   &(slave->dev->dev.kobj), "bonding_slave");
-	if (err) {
-		kobject_put(&slave->kobj);
-		return err;
-	}
-
 	for (a = slave_attrs; *a; ++a) {
 		err = sysfs_create_file(&slave->kobj, &((*a)->attr));
 		if (err) {
@@ -165,7 +176,7 @@ int bond_sysfs_slave_add(struct slave *slave)
 	return 0;
 }
 
-void bond_sysfs_slave_del(struct slave *slave)
+void bond_slave_kobj_put(struct slave *slave)
 {
 	const struct slave_attribute **a;
 
diff --git i/include/net/bonding.h w/include/net/bonding.h
index 7d132cc1e584..ccb07e3e495e 100644
--- i/include/net/bonding.h
+++ w/include/net/bonding.h
@@ -622,10 +622,12 @@ int bond_create(struct net *net, const char *name);
 int bond_create_sysfs(struct bond_net *net);
 void bond_destroy_sysfs(struct bond_net *net);
 void bond_prepare_sysfs_group(struct bonding *bond);
+int bond_slave_kobj_init(struct slave *slave);
 int bond_sysfs_slave_add(struct slave *slave);
-void bond_sysfs_slave_del(struct slave *slave);
+void bond_slave_kobj_put(struct slave *slave);
 int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 		 struct netlink_ext_ack *extack);
+void bond_free_slave(struct slave *slave);
 int bond_release(struct net_device *bond_dev, struct net_device *slave_dev);
 u32 bond_xmit_hash(struct bonding *bond, struct sk_buff *skb);
 int bond_set_carrier(struct bonding *bond);