Re: [PATCH] keys/keyctl: Use kfree_rcu instead of kfree

From: Greg KH <gregkh@linuxfoundation.org>
To: Siddh Raman Pant <code@siddh.me>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	James Morris <jmorris@namei.org>,
	David Howells <dhowells@redhat.com>,
	linux-security-modules <linux-security-module@vger.kernel.org>,
	keyrings <keyrings@vger.kernel.org>,
	linux-kernel-mentees
	<linux-kernel-mentees@lists.linuxfoundation.org>,
	"Serge E. Hallyn" <serge@hallyn.com>
Subject: Re: [PATCH] keys/keyctl: Use kfree_rcu instead of kfree
Date: Sat, 23 Jul 2022 16:43:11 +0200	[thread overview]
Message-ID: <YtwI/3/Qp3lSKuls@kroah.com> (raw)
In-Reply-To: <1822b7c129a.14411444236159.6380883938307880248@siddh.me>

On Sat, Jul 23, 2022 at 08:05:27PM +0530, Siddh Raman Pant wrote:
> On Sat, 23 Jul 2022 19:35:16 +0530  Greg KH <gregkh@linuxfoundation.org> wrote:
> > That does not explain why this change is needed.  What problem does this
> > solve?  Why use RCU if you don't have to?  What functionality did you
> > just change in this commit and why?
> 
> We can avoid a race condition wherein some process tries to access them while
> they are being freed. For instance, the comment on `watch_queue_clear()` also
> states that:
>         /*
>          * Remove all the watches that are contributory to a queue.  This has the
>          * potential to race with removal of the watches by the destruction of the
>          * objects being watched or with the distribution of notifications.
>          */
> And an RCU read critical section is initiated in that function, so we should
> use kfree_rcu() to not unintentionally free it while it is in the critical
> section.

You need to explain all of this in a changelog text.  Don't say what you
do, but say why you are doing it.

> > And how was this tested?
> 
> It compiles locally for me, and I used syzbot on this along with testing the
> other `watch_queue_clear` patch, which generated no errors.

How does the watch queue stuff relate to this keyctl logic?

Again, be specific as to why you are doing things.

thanks,

greg k-h
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