netdev - Re: [RFC PATCH net-next 1/2] devlink: Hold a reference on parent device

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Message-ID: <ZJgrOAmhIycWSKtU@shredder>
Date: Sun, 25 Jun 2023 14:55:36 +0300
From: Ido Schimmel <idosch@...dia.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
	pabeni@...hat.com, edumazet@...gle.com, petrm@...dia.com
Subject: Re: [RFC PATCH net-next 1/2] devlink: Hold a reference on parent
 device

On Thu, Jun 22, 2023 at 08:29:31AM +0200, Jiri Pirko wrote:
> Wed, Jun 21, 2023 at 05:35:15PM CEST, idosch@...dia.com wrote:
> >On Wed, Jun 21, 2023 at 01:48:36PM +0200, Jiri Pirko wrote:
> >> Mon, Jun 19, 2023 at 02:50:14PM CEST, idosch@...dia.com wrote:
> >> >@@ -91,6 +92,7 @@ static void devlink_release(struct work_struct *work)
> >> > 
> >> > 	mutex_destroy(&devlink->lock);
> >> > 	lockdep_unregister_key(&devlink->lock_key);
> >> >+	put_device(devlink->dev);
> >> 
> >> In this case I think you have to make sure this is called before
> >> devlink_free() ends. After the caller of devlink_free() returns (most
> >> probably .remove callback), nothing stops module from being removed.
> >> 
> >> I don't see other way. Utilize complete() here and wait_for_completion()
> >> at the end of devlink_free().
> >
> >I might be missing something, but how can I do something like
> >wait_for_completion(&devlink->comp) at the end of devlink_free()? After
> >I call devlink_put() the devlink instance can be freed and the
> >wait_for_completion() call will result in a UAF.
> 
> You have to move the free() to devlink_free()
> Basically, all the things done in devlink_put that are symmetrical to
> the initialization done in devlink_alloc() should be moved there.

But it's a bit weird to dereference 'devlink' (to wait for the
completion) after calling 'devlink_put(devlink)'. Given that this
problem seems to be specific to netdevsim, don't you think it's better
to fix it in netdevsim rather than working around it in devlink?

> 
> 
> >
> >> 
> >> If the completion in devlink_put() area rings a bell for you, let me save
> >> you the trouble looking it up:
> >> 9053637e0da7 ("devlink: remove the registration guarantee of references")
> >> This commit removed that. But it is a different usage.
> >> 
> >> 
> >> 
> >> > 	kfree(devlink);
> >> > }
> >> > 
> >> >@@ -204,6 +206,7 @@ struct devlink *devlink_alloc_ns(const struct devlink_ops *ops,
> >> > 	if (ret < 0)
> >> > 		goto err_xa_alloc;
> >> > 
> >> >+	get_device(dev);
> >> > 	devlink->dev = dev;
> >> > 	devlink->ops = ops;
> >> > 	xa_init_flags(&devlink->ports, XA_FLAGS_ALLOC);
> >> >-- 
> >> >2.40.1
> >> >