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

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Date:   Mon, 24 Oct 2016 08:36:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     viro@...iv.linux.org.uk
Cc:     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)

Al,

any chance to send this user triggerable use after free on to Linus?

On Sun, Oct 16, 2016 at 07:51:22AM +0200, Christoph Hellwig wrote:
> 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>
> ---
>  fs/aio.c           | 28 +++++++++++++++++++++++++---
>  include/linux/fs.h |  1 +
>  2 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 1157e13..bf315cd 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1078,6 +1078,17 @@ static void aio_complete(struct kiocb *kiocb, long res, long res2)
>  	unsigned tail, pos, head;
>  	unsigned long	flags;
>  
> +	if (kiocb->ki_flags & IOCB_WRITE) {
> +		struct file *file = kiocb->ki_filp;
> +
> +		/*
> +		 * Tell lockdep we inherited freeze protection from submission
> +		 * thread.
> +		 */
> +		__sb_writers_acquired(file_inode(file)->i_sb, SB_FREEZE_WRITE);
> +		file_end_write(file);
> +	}
> +
>  	/*
>  	 * Special case handling for sync iocbs:
>  	 *  - events go directly into the iocb for fast handling
> @@ -1460,13 +1471,24 @@ static ssize_t aio_run_iocb(struct kiocb *req, unsigned opcode,
>  			return ret;
>  		}
>  
> -		if (rw == WRITE)
> +		if (rw == WRITE) {
>  			file_start_write(file);
> +			req->ki_flags |= IOCB_WRITE;
> +		}
> +
> +		if (rw == WRITE) {
> +			/*
> +			 * We release freeze protection in aio_complete(). Fool
> +			 * lockdep by telling it the lock got released so that
> +			 * it doesn't complain about held lock when we return
> +			 * to userspace.
> +			 */
> +			__sb_writers_release(file_inode(file)->i_sb,
> +					SB_FREEZE_WRITE);
> +		}
>  
>  		ret = iter_op(req, &iter);
>  
> -		if (rw == WRITE)
> -			file_end_write(file);
>  		kfree(iovec);
>  		break;
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 16d2b6e..db600e9 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -321,6 +321,7 @@ struct writeback_control;
>  #define IOCB_HIPRI		(1 << 3)
>  #define IOCB_DSYNC		(1 << 4)
>  #define IOCB_SYNC		(1 << 5)
> +#define IOCB_WRITE		(1 << 6)
>  
>  struct kiocb {
>  	struct file		*ki_filp;
> -- 
> 2.1.4
> 
> --
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