[PATCH 2/4] xfs: reject completely bogus remount options

[Dave Chinner]

On Fri, Oct 11, 2013 at 02:11:18PM -0500, Eric Sandeen wrote:
> There's a long comment about handling non-remountable
> options in xfs_fs_remount, but nothing addresses the case
> of completely bogus mount options at remount time, which
> can lead to some severe strangeness:
> 
> # for I in `seq 1 10`; do mount -o remount,noacl /mnt/test2; done
> # for I in `seq 1 10`; do mount -o remount,badoption /mnt/test2; done
> # grep sdb4 /etc/mtab
> /dev/sdb4 /mnt/test2 xfs rw,noacl,noacl,noacl,noacl,noacl,noacl,noacl,noacl,noacl,noacl,noacl,badoption,badoption,badoption,badoption,badoption,badoption,badoption,badoption,badoption,badoption 0 0
> 
> This is a bit of a hack, but we can re-use xfs_parseargs()
> with a dummy mount struct to just vet all of the remount
> options which were passed in.  With this, we get a saner
> result:
> 
> [44898.102990] EXT4-fs (sdb4): Unrecognized mount option "badoption" or missing value

ext4? Really? :)

> +++ b/fs/xfs/xfs_super.c
> @@ -1202,11 +1202,25 @@ xfs_fs_remount(
>  	int			*flags,
>  	char			*options)
>  {
> -	struct xfs_mount	*mp = XFS_M(sb);
> +	struct xfs_mount	*mp = XFS_M(sb), *dummy_mp;
>  	substring_t		args[MAX_OPT_ARGS];
>  	char			*p;
>  	int			error;
>  
> +	/*
> +	 * Check all the mount options presented to be sure
> +	 * there's nothing too crazy in there.  Non-remountable
> +	 * but valid options are a different issue.
> +	 */
> +	dummy_mp = kmem_zalloc(sizeof(*dummy_mp), KM_MAYFAIL);
> +	if (dummy_mp) {
> +		dummy_mp->m_super = sb;
> +		error = xfs_parseargs(dummy_mp, options);
> +		kfree(dummy_mp);
> +		if (error)
> +			return -error;

This, at minimum, leaks dummy_mp->m_fsname, and it will leak other
strings that are also kstrdup()d by xfs_parseargs().

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com