xfs: only return detailed fsmap info if the caller has CAP_SYS_ADMIN

There were a number of handwaving complaints that one could "possibly"
use inode numbers and extent maps to fingerprint a filesystem hosting
multiple containers and somehow use the information to guess at the
contents of other containers and attack them.  Despite the total lack of
any demonstration that this is actually possible, it's easier to
restrict access now and broaden it later, so use the rmapbt fsmap
backends only if the caller has CAP_SYS_ADMIN.  Unprivileged users will
just have to make do with only getting the free space and static
metadata placement information.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

fs/xfs/xfs_fsmap.c

@@ -828,6 +828,7 @@ xfs_getfsmap(
 	struct xfs_fsmap		dkeys[2];	/* per-dev keys */
 	struct xfs_getfsmap_dev		handlers[XFS_GETFSMAP_DEVS];
 	struct xfs_getfsmap_info	info = { NULL };
+	bool				use_rmap;
 	int				i;
 	int				error = 0;
 
@@ -837,12 +838,14 @@ xfs_getfsmap(
 	    !xfs_getfsmap_is_valid_device(mp, &head->fmh_keys[1]))
 		return -EINVAL;
 
+	use_rmap = capable(CAP_SYS_ADMIN) &&
+		   xfs_sb_version_hasrmapbt(&mp->m_sb);
 	head->fmh_entries = 0;
 
 	/* Set up our device handlers. */
 	memset(handlers, 0, sizeof(handlers));
 	handlers[0].dev = new_encode_dev(mp->m_ddev_targp->bt_dev);
-	if (xfs_sb_version_hasrmapbt(&mp->m_sb))
+	if (use_rmap)
 		handlers[0].fn = xfs_getfsmap_datadev_rmapbt;
 	else
 		handlers[0].fn = xfs_getfsmap_datadev_bnobt;