linux-kernel - Re: [PATCH] keys/keyctl: Use kfree

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Message-ID: <YtwI/3/Qp3lSKuls@kroah.com>
Date:   Sat, 23 Jul 2022 16:43:11 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Siddh Raman Pant <code@...dh.me>
Cc:     David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-modules <linux-security-module@...r.kernel.org>,
        keyrings <keyrings@...r.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] keys/keyctl: Use kfree_rcu instead of kfree

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@...uxfoundation.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