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

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Message-ID: <20161017185552.GA24653@lst.de>
Date:   Mon, 17 Oct 2016 20:55:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, 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,
        Peter Zijlstra <peterz@...radead.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 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..