linux-kernel - Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

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Date:   Mon, 17 Oct 2016 21:54:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Jeff Moyer <jmoyer@...hat.com>, viro@...iv.linux.org.uk,
        jack@...e.cz, dmonakhov@...nvz.org, linux-fsdevel@...r.kernel.org,
        linux-aio@...ck.org, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [PATCH] aio: fix a use after free (and fix freeze protection of
 aio writes)

On Mon, Oct 17, 2016 at 08:55:52PM +0200, Christoph Hellwig wrote:
> On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote:
> > This ends up being a call to __sb_end_write:
> > 
> > void __sb_end_write(struct super_block *sb, int level)
> > {
> >         percpu_up_read(sb->s_writers.rw_sem + level-1);
> > }
> > 
> > Nothing guarantees that submission and completion happen on the same
> > CPU.  Is this safe?
> 
> Good point.  From my reading of the percpu_rwsem implementation it
> is not safe to release it from a different CPU.  Which makes me
> wonder how we can protect aio writes properly here..

percpu-rwsem has the same semantics as regular rwsems, so preemptible
and 'owner' stuff.

Therefore we must support doing up from a different cpu than we did down
on; the owner could've been migrated while we held it.

And while there's a metric ton of tricky in the implementation, this
part is actually fairly straight forward. We only care about the direct
sum of the per-cpu counter, see readers_active_check() -> per_cpu_sum().

So one cpu doing an inc and another doing a dec summed is still 0.