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

[Eric Dumazet <eric.dumazet@gmail.com>]

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 <wu000273@umn.edu>
> Cc: Jay Vosburgh <j.vosburgh@gmail.com>
> Cc: Veaceslav Falico <vfalico@gmail.com>
> Cc: Andy Gospodarek <andy@greyhouse.net>
> Signed-off-by: Jamie Iles <jamie@nuviainc.com>
> ---
>  drivers/net/bonding/bond_sysfs_slave.c | 12 ++++++++++++
>  include/net/bonding.h                  |  2 ++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/drivers/net/bonding/bond_sysfs_slave.c b/drivers/net/bonding/bond_sysfs_slave.c
> index 9b8346638f69..2fdbcf9692c5 100644
> --- a/drivers/net/bonding/bond_sysfs_slave.c
> +++ b/drivers/net/bonding/bond_sysfs_slave.c
> @@ -136,7 +136,15 @@ static const struct sysfs_ops slave_sysfs_ops = {
>  	.show = slave_show,
>  };
>  
> +static void slave_release(struct kobject *kobj)
> +{
> +	struct slave *slave = to_slave(kobj);
> +
> +	complete(&slave->kobj_unregister_done);
> +}
> +
>  static struct kobj_type slave_ktype = {
> +	.release = slave_release,
>  #ifdef CONFIG_SYSFS
>  	.sysfs_ops = &slave_sysfs_ops,
>  #endif
> @@ -147,10 +155,12 @@ int bond_sysfs_slave_add(struct slave *slave)
>  	const struct slave_attribute **a;
>  	int err;
>  
> +	init_completion(&slave->kobj_unregister_done);
>  	err = kobject_init_and_add(&slave->kobj, &slave_ktype,
>  				   &(slave->dev->dev.kobj), "bonding_slave");
>  	if (err) {
>  		kobject_put(&slave->kobj);
> +		wait_for_completion(&slave->kobj_unregister_done);
>  		return err;
>  	}
>  
> @@ -158,6 +168,7 @@ int bond_sysfs_slave_add(struct slave *slave)
>  		err = sysfs_create_file(&slave->kobj, &((*a)->attr));
>  		if (err) {
>  			kobject_put(&slave->kobj);
> +			wait_for_completion(&slave->kobj_unregister_done);
>  			return err;
>  		}
>  	}
> @@ -173,4 +184,5 @@ void bond_sysfs_slave_del(struct slave *slave)
>  		sysfs_remove_file(&slave->kobj, &((*a)->attr));
>  
>  	kobject_put(&slave->kobj);
> +	wait_for_completion(&slave->kobj_unregister_done);
>  }
> diff --git a/include/net/bonding.h b/include/net/bonding.h
> index 7d132cc1e584..78d771d2ffd3 100644
> --- a/include/net/bonding.h
> +++ b/include/net/bonding.h
> @@ -25,6 +25,7 @@
>  #include <linux/etherdevice.h>
>  #include <linux/reciprocal_div.h>
>  #include <linux/if_link.h>
> +#include <linux/completion.h>
>  
>  #include <net/bond_3ad.h>
>  #include <net/bond_alb.h>
> @@ -182,6 +183,7 @@ struct slave {
>  #endif
>  	struct delayed_work notify_work;
>  	struct kobject kobj;
> +	struct completion kobj_unregister_done;
>  	struct rtnl_link_stats64 slave_stats;
>  };


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.