[PATCH v2] mtd: fix use-after-free in mtd release

[Alexander Usyskin]

I case of partition device_unregister() in mtd_device_release()
calls mtd_release() which frees mtd_info structure for partition.
All code after device_unregister in mtd_device_release thus
uses already freed memory.

Move part of code to mtd_release() and restict mtd->dev cleanup
to non-partion object.
For partition object such cleanup have no sense as partition
mtd_info is removed.

Cc: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
Cc: Zhang Xiaoxu <zhangxiaoxu5@xxxxxxxxxx>
Fixes: 19bfa9ebebb5 ("mtd: use refcount to prevent corruption")
Reviewed-by: Tomas Winkler <tomas.winkler@xxxxxxxxx>
Signed-off-by: Alexander Usyskin <alexander.usyskin@xxxxxxxxx>
---

V2: Fix commit messsge wording

---
 drivers/mtd/mtdcore.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 2466ea466466..46f15f676491 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -93,6 +93,9 @@ static void mtd_release(struct device *dev)
 	struct mtd_info *mtd = dev_get_drvdata(dev);
 	dev_t index = MTD_DEVT(mtd->index);
 
+	idr_remove(&mtd_idr, mtd->index);
+	of_node_put(mtd_get_of_node(mtd));
+
 	if (mtd_is_partition(mtd))
 		release_mtd_partition(mtd);
 
@@ -103,6 +106,7 @@ static void mtd_release(struct device *dev)
 static void mtd_device_release(struct kref *kref)
 {
 	struct mtd_info *mtd = container_of(kref, struct mtd_info, refcnt);
+	bool is_partition = mtd_is_partition(mtd);
 
 	debugfs_remove_recursive(mtd->dbg.dfs_dir);
 
@@ -111,11 +115,13 @@ static void mtd_device_release(struct kref *kref)
 
 	device_unregister(&mtd->dev);
 
-	/* Clear dev so mtd can be safely re-registered later if desired */
-	memset(&mtd->dev, 0, sizeof(mtd->dev));
-
-	idr_remove(&mtd_idr, mtd->index);
-	of_node_put(mtd_get_of_node(mtd));
+	/*
+	 *  Clear dev so mtd can be safely re-registered later if desired.
+	 *  Should not be done for partition,
+	 *  as it was already destroyed in device_unregister().
+	 */
+	if (!is_partition)
+		memset(&mtd->dev, 0, sizeof(mtd->dev));
 
 	module_put(THIS_MODULE);
 }
-- 
2.34.1