linux-kernel - Re: [PATCH] ext4: avoid exposure of stale data in ext4_punch

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Date:	Thu, 26 Sep 2013 20:53:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	Maxim Patlasov <MPatlasov@...allels.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org,
	adilger.kernel@...ger.ca, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: avoid exposure of stale data in ext4_punch_hole()

  Hello,

On Thu 26-09-13 21:32:07, Maxim Patlasov wrote:
> While handling punch-hole fallocate, it's useless to truncate page cache
> before removing the range from extent tree (or block map in indirect case)
> because page cache can be re-populated (by read-ahead or read(2) or mmap-ed
> read) immediately after truncating page cache, but before updating extent
> tree (or block map). In that case the user will see stale data even after
> fallocate is completed.
  Yes, this is a known problem. The trouble is there isn't a reliable fix
currently possible. If we don't truncate page cache before removing blocks,
we will have pages in memory being backed by already freed blocks - not
good as that can lead to data corruption. So you should't really remove the
truncation from before we remove the blocks.

You are right that if punch hole races with page fault or read, we can
create again pages with block mapping which will become stale soon and the
same problem as I wrote above applies. Truncating pagecache after we
removed blocks only narrows the race window but doesn't really fix the
problem.

Properly fixing the problem requires significant overhaul in how mmap_sem
is used in page fault. I'm working on patches to do that but it will take
some time.

								Honza
 
> Signed-off-by: Maxim Patlasov <mpatlasov@...allels.com>
> ---
>  fs/ext4/inode.c |   17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0d424d7..6b71116 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3564,14 +3564,6 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
>  
>  	}
>  
> -	first_block_offset = round_up(offset, sb->s_blocksize);
> -	last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
> -
> -	/* Now release the pages and zero block aligned part of pages*/
> -	if (last_block_offset > first_block_offset)
> -		truncate_pagecache_range(inode, first_block_offset,
> -					 last_block_offset);
> -
>  	/* Wait all existing dio workers, newcomers will block on i_mutex */
>  	ext4_inode_block_unlocked_dio(inode);
>  	inode_dio_wait(inode);
> @@ -3621,6 +3613,15 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
>  	up_write(&EXT4_I(inode)->i_data_sem);
>  	if (IS_SYNC(inode))
>  		ext4_handle_sync(handle);
> +
> +	first_block_offset = round_up(offset, sb->s_blocksize);
> +	last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
> +
> +	/* Now release the pages and zero block aligned part of pages */
> +	if (last_block_offset > first_block_offset)
> +		truncate_pagecache_range(inode, first_block_offset,
> +					 last_block_offset);
> +
>  	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
>  	ext4_mark_inode_dirty(handle, inode);
>  out_stop:
> 
> --
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-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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