Re: [PATCH 3/3] xfs: Fix stale data exposure when readahead races with hole punch

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On Thu, Aug 29, 2019 at 03:10:34PM +0200, Jan Kara wrote:
> Hole puching currently evicts pages from page cache and then goes on to
> remove blocks from the inode. This happens under both XFS_IOLOCK_EXCL
> and XFS_MMAPLOCK_EXCL which provides appropriate serialization with
> racing reads or page faults. However there is currently nothing that
> prevents readahead triggered by fadvise() or madvise() from racing with
> the hole punch and instantiating page cache page after hole punching has
> evicted page cache in xfs_flush_unmap_range() but before it has removed
> blocks from the inode. This page cache page will be mapping soon to be
> freed block and that can lead to returning stale data to userspace or
> even filesystem corruption.
> 
> Fix the problem by protecting handling of readahead requests by
> XFS_IOLOCK_SHARED similarly as we protect reads.
> 
> CC: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahcyeaEVOFKVQ5dw@xxxxxxxxxxxxxx/
> Reported-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>

Is there a test on xfstests to demonstrate this race?

Will test it out though...

Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

--D

> ---
>  fs/xfs/xfs_file.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 28101bbc0b78..d952d5962e93 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -28,6 +28,7 @@
>  #include <linux/falloc.h>
>  #include <linux/backing-dev.h>
>  #include <linux/mman.h>
> +#include <linux/fadvise.h>
>  
>  static const struct vm_operations_struct xfs_file_vm_ops;
>  
> @@ -933,6 +934,30 @@ xfs_file_fallocate(
>  	return error;
>  }
>  
> +STATIC int
> +xfs_file_fadvise(
> +	struct file	*file,
> +	loff_t		start,
> +	loff_t		end,
> +	int		advice)
> +{
> +	struct xfs_inode *ip = XFS_I(file_inode(file));
> +	int ret;
> +	int lockflags = 0;
> +
> +	/*
> +	 * Operations creating pages in page cache need protection from hole
> +	 * punching and similar ops
> +	 */
> +	if (advice == POSIX_FADV_WILLNEED) {
> +		lockflags = XFS_IOLOCK_SHARED;
> +		xfs_ilock(ip, lockflags);
> +	}
> +	ret = generic_fadvise(file, start, end, advice);
> +	if (lockflags)
> +		xfs_iunlock(ip, lockflags);
> +	return ret;
> +}
>  
>  STATIC loff_t
>  xfs_file_remap_range(
> @@ -1232,6 +1257,7 @@ const struct file_operations xfs_file_operations = {
>  	.fsync		= xfs_file_fsync,
>  	.get_unmapped_area = thp_get_unmapped_area,
>  	.fallocate	= xfs_file_fallocate,
> +	.fadvise	= xfs_file_fadvise,
>  	.remap_file_range = xfs_file_remap_range,
>  };
>  
> -- 
> 2.16.4
> 



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