mtd: key superblock by device number

The mtd driver has similar problems than the one that was fixed in
commit dc3216b1 ("super: ensure valid info").

The kill_mtd_super() helper calls shuts the superblock down but leaves
the superblock on fs_supers as the devices are still in use but puts the
mtd device and cleans out the superblock's s_mtd field.

This means another mounter can find the superblock on the list accessing
its s_mtd field while it is curently in the process of being freed or
already freed.

Prevent that from happening by keying superblock by dev_t just as we do
in the generic code.

Link: https://lore.kernel.org/linux-fsdevel/20230829-weitab-lauwarm-49c40fc85863@braunerAcked-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230829-vfs-super-mtd-v1-2-fecb572e5df3@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

drivers/mtd/mtdsuper.c

@@ -19,38 +19,6 @@
 #include <linux/fs_context.h>
 #include "mtdcore.h"
 
-/*
- * compare superblocks to see if they're equivalent
- * - they are if the underlying MTD device is the same
- */
-static int mtd_test_super(struct super_block *sb, struct fs_context *fc)
-{
-	struct mtd_info *mtd = fc->sget_key;
-
-	if (sb->s_mtd == fc->sget_key) {
-		pr_debug("MTDSB: Match on device %d (\"%s\")\n",
-			 mtd->index, mtd->name);
-		return 1;
-	}
-
-	pr_debug("MTDSB: No match, device %d (\"%s\"), device %d (\"%s\")\n",
-		 sb->s_mtd->index, sb->s_mtd->name, mtd->index, mtd->name);
-	return 0;
-}
-
-/*
- * mark the superblock by the MTD device it is using
- * - set the device number to be the correct MTD block device for pesuperstence
- *   of NFS exports
- */
-static int mtd_set_super(struct super_block *sb, struct fs_context *fc)
-{
-	sb->s_mtd = fc->sget_key;
-	sb->s_dev = MKDEV(MTD_BLOCK_MAJOR, sb->s_mtd->index);
-	sb->s_bdi = bdi_get(mtd_bdi);
-	return 0;
-}
-
 /*
  * get a superblock on an MTD-backed filesystem
  */
@@ -62,8 +30,7 @@ static int mtd_get_sb(struct fs_context *fc,
 	struct super_block *sb;
 	int ret;
 
-	fc->sget_key = mtd;
-	sb = sget_fc(fc, mtd_test_super, mtd_set_super);
+	sb = sget_dev(fc, MKDEV(MTD_BLOCK_MAJOR, mtd->index));
 	if (IS_ERR(sb))
 		return PTR_ERR(sb);
 
@@ -77,6 +44,16 @@ static int mtd_get_sb(struct fs_context *fc,
 		pr_debug("MTDSB: New superblock for device %d (\"%s\")\n",
 			 mtd->index, mtd->name);
 
+		/*
+		 * Would usually have been set with @sb_lock held but in
+		 * contrast to sb->s_bdev that's checked with only
+		 * @sb_lock held, nothing checks sb->s_mtd without also
+		 * holding sb->s_umount and we're holding sb->s_umount
+		 * here.
+		 */
+		sb->s_mtd = mtd;
+		sb->s_bdi = bdi_get(mtd_bdi);
+
 		ret = fill_super(sb, fc);
 		if (ret < 0)
 			goto error_sb;