Patchwork [4/9] ext4: punch_hole should wait for DIO writers

punch_hole are the places where we have to wait for all existing writers
(writeback, aio, dio), but currently we simply flush pended end_io request
which is not sufficient. Even more i_mutex is not holded while punch_hole
which obviously result in dangerous data corruption due to
access-after-free issue.

This patch performs following changes:
- Guard punch_hole with i_mutex
- Block all new dio readers in order to prevent information leak caused by
  read-after-free pattern.
- punch_hole now wait for all writers in flight
  NOTE: XXX write-after-free race is still possible because there is
        no easy way to stop writeback while punch_hole is in progress.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/ext4/extents.c |   41 +++++++++++++++++++++++++----------------
 1 files changed, 25 insertions(+), 16 deletions(-)

On Thu, 13 Sep 2012, Dmitry Monakhov wrote:

> Date: Thu, 13 Sep 2012 19:01:09 +0400
> From: Dmitry Monakhov <dmonakhov@openvz.org>
> To: linux-ext4@vger.kernel.org
> Cc: tytso@mit.edu, jack@suse.cz, wenqing.lz@taobao.com,
>     Dmitry Monakhov <dmonakhov@openvz.org>
> Subject: [PATCH 4/9] ext4: punch_hole should wait for DIO writers
> 
> punch_hole are the places where we have to wait for all existing writers
> (writeback, aio, dio), but currently we simply flush pended end_io request
> which is not sufficient. Even more i_mutex is not holded while punch_hole
> which obviously result in dangerous data corruption due to
> access-after-free issue.
> 
> This patch performs following changes:
> - Guard punch_hole with i_mutex
> - Block all new dio readers in order to prevent information leak caused by
>   read-after-free pattern.
> - punch_hole now wait for all writers in flight
>   NOTE: XXX write-after-free race is still possible because there is
>         no easy way to stop writeback while punch_hole is in progress.

Hi Dimitry,

Just FYI, I am carrying the punch hole i_mutex in the patch set "Add
invalidatepage_range address space operation" but I am more than
happy to drop it and let your patch fix this instead.

Thanks!
-Lukas


> 
> Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> ---
>  fs/ext4/extents.c |   41 +++++++++++++++++++++++++----------------
>  1 files changed, 25 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 44e33b0..0e94485 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4814,9 +4814,22 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  	loff_t first_page_offset, last_page_offset;
>  	int credits, err = 0;
>  
> +	/*
> +	 * Write out all dirty pages to avoid race conditions
> +	 * Then release them.
> +	 */
> +	if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
> +		err = filemap_write_and_wait_range(mapping,
> +			offset, offset + length - 1);
> +
> +		if (err)
> +			return err;
> +	}
> +
> +	mutex_lock(&inode->i_mutex);
>  	/* No need to punch hole beyond i_size */
>  	if (offset >= inode->i_size)
> -		return 0;
> +		goto out_mutex;
>  
>  	/*
>  	 * If the hole extends beyond i_size, set the hole
> @@ -4834,31 +4847,23 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  	first_page_offset = first_page << PAGE_CACHE_SHIFT;
>  	last_page_offset = last_page << PAGE_CACHE_SHIFT;
>  
> -	/*
> -	 * Write out all dirty pages to avoid race conditions
> -	 * Then release them.
> -	 */
> -	if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
> -		err = filemap_write_and_wait_range(mapping,
> -			offset, offset + length - 1);
> -
> -		if (err)
> -			return err;
> -	}
> -
>  	/* Now release the pages */
>  	if (last_page_offset > first_page_offset) {
>  		truncate_pagecache_range(inode, first_page_offset,
>  					 last_page_offset - 1);
>  	}
>  
> -	/* finish any pending end_io work */
> +	/* Wait all existing dio workers, newcomers will block on i_mutex */
> +	ext4_inode_block_unlocked_dio(inode);
> +	inode_dio_wait(inode);
>  	ext4_flush_completed_IO(inode);
>  
>  	credits = ext4_writepage_trans_blocks(inode);
>  	handle = ext4_journal_start(inode, credits);
> -	if (IS_ERR(handle))
> -		return PTR_ERR(handle);
> +	if (IS_ERR(handle)) {
> +		err = PTR_ERR(handle);
> +		goto out_dio;
> +	}
>  
>  	err = ext4_orphan_add(handle, inode);
>  	if (err)
> @@ -4952,6 +4957,10 @@ out:
>  	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
>  	ext4_mark_inode_dirty(handle, inode);
>  	ext4_journal_stop(handle);
> +out_dio:
> +	ext4_inode_resume_unlocked_dio(inode);
> +out_mutex:
> +	mutex_unlock(&inode->i_mutex);
>  	return err;
>  }
>  int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> 
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