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 15:40:24 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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)

Christoph Hellwig <hch@....de> 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..

Could we just change percpu_rw_semaphore->read_count to be a signed
integer?  The down_write path sums up the counters from all cpus...

-Jeff