linux-kernel - Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

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Date:   Mon, 14 Jan 2019 08:52:33 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

On Mon, 2019-01-14 at 13:52 +0100, Peter Zijlstra wrote:
> On Fri, Jan 11, 2019 at 09:01:41AM -0800, Bart Van Assche wrote:
> > The list_del_rcu() call must only happen once. 
> 
> Yes; obviously. But if we need to check all @pf's, that means the entry
> is still reachable after a single reset_lock()/free_key_range(), which
> is a bug.
> 
> > I ran into complaints reporting that
> > the list_del_rcu() call triggered list corruption. This change made these complaints
> > disappear.
> 
> I'm saying this solution buggy, because that means the entry is still
> reachable after we do call_rcu() (which is a straight up UAF).
> 
> Also put it differently, what guarantees checking those two @pf's is
> sufficient. Suppose your earlier @pf already did the RCU callback and
> freed stuff while the second is in progress. Then you're poking into
> dead space.

zap_class() only examines elements of the list_entries[] array for which the
corresponding bit in list_entries_in_use has been set. The RCU callback clears 
the bits in the list_entries_in_use that correspond to elements that have been
freed. The graph lock serializes zap_class() calls and the code inside the
RCU callback. So it's not clear to me why you are claiming that zap_class()
would trigger a use-after-free?

Bart.