linux-kernel - Re: [PATCH v2 1/3] seccomp: Add find

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Message-ID: <202005291309.6183EC0604@keescook>
Date:   Fri, 29 May 2020 13:14:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     christian.brauner@...ntu.com,
        containers@...ts.linux-foundation.org, cyphar@...har.com,
        jannh@...gle.com, jeffv@...gle.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, palmer@...gle.com, rsesek@...gle.com,
        tycho@...ho.ws, Matt Denton <mpdenton@...gle.com>
Subject: Re: [PATCH v2 1/3] seccomp: Add find_notification helper

On Fri, May 29, 2020 at 05:40:38PM +0000, Sargun Dhillon wrote:
> > 
> > While the comment is good, let's actually enforce this with:
> > 
> > if (WARN_ON(!mutex_is_locked(&filter->notif_lock)))
> > 	return NULL;
> > 
> I don't see much use of lockdep in seccomp (well, any), but
> wouldn't a stronger statement be to use lockdep, and just have:
> 
> lockdep_assert_held(&filter->notify_lock);
> 
> As that checks that the lock is held by the current task.

/me slaps his forehead

Yes. I need more coffee or something. Yes, I meant
lockdep_assert_held(), and now I need to go fix my pstore series since I
confused myself into the wrong function and using it so many times in
pstore overwrote the correct function in my head. Thank you!

> Although, that does put this check behind lockdep, which means
> that running in "normal" circumstances is less safe (but faster?).

Now, that's fine. The check needs to be "am *I* holding this mutex?" and
I don't think anything except lockdep can do that.

-- 
Kees Cook