Re: [PATCH] fix bmap-vs-truncate race

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, Mar 30, 2009 at 03:20:24PM -0400, Mikulas Patocka wrote:
> Hi
> 
> I'm submitting this patch for 2.6.30 merge window.
> 
> 
> FAT filesystem used down_read(&mapping->host->i_alloc_sem) to prevent 
> a race between bmap and truncate. However, such race is present in all the
> other filesystems --- it is generally assumed that blocks queried with
> get_block won't disappear while get_block is in progress.
> 
> The race can be only triggered by root, non-privileged users can't use
> bmap, so it is not a security issue (unless there is some program run
> by root that bmaps users' files).
> 
> This patch fixes the race in a generic way, in all the filesystems. If some
> filesystem employs its own locking and doesn't want to take i_alloc_sem
> (I don't know about any, where taking i_alloc_sem could be problem),
> let it use its own function and not generic_block_bmap.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  fs/buffer.c    |    8 ++++++++
>  fs/fat/inode.c |    2 --
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6.29-devel/fs/buffer.c
> ===================================================================
> --- linux-2.6.29-devel.orig/fs/buffer.c	2009-03-29 04:48:38.000000000 +0200
> +++ linux-2.6.29-devel/fs/buffer.c	2009-03-29 05:26:53.000000000 +0200
> @@ -2963,7 +2963,15 @@ sector_t generic_block_bmap(struct addre
>  	tmp.b_state = 0;
>  	tmp.b_blocknr = 0;
>  	tmp.b_size = 1 << inode->i_blkbits;
> +
> +	/*
> +	 * Protect the inode from being truncated while get_block is
> +	 * in progress.
> +	 */
> +	down_read(&mapping->host->i_alloc_sem);
>  	get_block(inode, block, &tmp, 0);
> +	up_read(&mapping->host->i_alloc_sem);
> +
>  	return tmp.b_blocknr;
>  }
> 
> Index: linux-2.6.29-devel/fs/fat/inode.c
> ===================================================================
> --- linux-2.6.29-devel.orig/fs/fat/inode.c	2009-03-29 04:46:35.000000000 +0200
> +++ linux-2.6.29-devel/fs/fat/inode.c	2009-03-29 05:26:53.000000000 +0200
> @@ -202,9 +202,7 @@ static sector_t _fat_bmap(struct address
>  	sector_t blocknr;
> 
>  	/* fat_get_cluster() assumes the requested blocknr isn't truncated. */
> -	down_read(&mapping->host->i_alloc_sem);
>  	blocknr = generic_block_bmap(mapping, block, fat_get_block);
> -	up_read(&mapping->host->i_alloc_sem);
> 
>  	return blocknr;
>  }

ext4_inode_info.i_data_sem should protect against get_block and truncate
race . On ext3 ext3_inode_info.truncate_mutex should do the same. We
already take these locks in get_block

-aneesh