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

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Message-ID: <x49wph6u4fe.fsf@segfault.boston.devel.redhat.com>
Date:   Mon, 17 Oct 2016 16:05:41 -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
Subject: Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

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

> From: Jan Kara <jack@...e.cz>
>
> Currently we dropped freeze protection of aio writes just after IO was
> submitted. Thus aio write could be in flight while the filesystem was
> frozen and that could result in unexpected situation like aio completion
> wanting to convert extent type on frozen filesystem. Testcase from
> Dmitry triggering this is like:
>
> for ((i=0;i<60;i++));do fsfreeze -f /mnt ;sleep 1;fsfreeze -u /mnt;done &
> fio --bs=4k --ioengine=libaio --iodepth=128 --size=1g --direct=1 \
>     --runtime=60 --filename=/mnt/file --name=rand-write --rw=randwrite
>
> Fix the problem by dropping freeze protection only once IO is completed
> in aio_complete().
>
> [hch: The above was the changelog of the original patch from Jan.
>  It turns out that it fixes something even more important - a use
>  after free of the file structucture given that the direct I/O
>  code calls fput and potentially drops the last reference to it in
>  aio_complete.  Together with two racing threads and a zero sized
>  I/O this seems easily exploitable]
>
> Reported-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> Signed-off-by: Jan Kara <jack@...e.cz>
> [hch: switch to use __sb_writers_acquired and file_inode(file),
>       updated changelog]
> Signed-off-by: Christoph Hellwig <hch@....de>

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>