pNFS/flexfiles: Check the layout validity in ff_layout_mirror_prepare_stats

Ensure that we check the layout pointer and validity after dereferencing
it in ff_layout_mirror_prepare_stats.

Fixes: 08e2e5bc ("pNFS/flexfiles: Clean up layoutstats")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -2520,9 +2520,9 @@ ff_layout_mirror_prepare_stats(struct pnfs_layout_hdr *lo,
 	return i;
 }
 
-static int
-ff_layout_prepare_layoutstats(struct nfs42_layoutstat_args *args)
+static int ff_layout_prepare_layoutstats(struct nfs42_layoutstat_args *args)
 {
+	struct pnfs_layout_hdr *lo;
 	struct nfs4_flexfile_layout *ff_layout;
 	const int dev_count = PNFS_LAYOUTSTATS_MAXDEV;
 
@@ -2533,11 +2533,14 @@ ff_layout_prepare_layoutstats(struct nfs42_layoutstat_args *args)
 		return -ENOMEM;
 
 	spin_lock(&args->inode->i_lock);
-	ff_layout = FF_LAYOUT_FROM_HDR(NFS_I(args->inode)->layout);
-	args->num_dev = ff_layout_mirror_prepare_stats(&ff_layout->generic_hdr,
-						       &args->devinfo[0],
-						       dev_count,
-						       NFS4_FF_OP_LAYOUTSTATS);
+	lo = NFS_I(args->inode)->layout;
+	if (lo && pnfs_layout_is_valid(lo)) {
+		ff_layout = FF_LAYOUT_FROM_HDR(lo);
+		args->num_dev = ff_layout_mirror_prepare_stats(
+			&ff_layout->generic_hdr, &args->devinfo[0], dev_count,
+			NFS4_FF_OP_LAYOUTSTATS);
+	} else
+		args->num_dev = 0;
 	spin_unlock(&args->inode->i_lock);
 	if (!args->num_dev) {
 		kfree(args->devinfo);