mtd: key superblock by device number

The mtd driver has similar problems than the one that was fixed in
commit dc3216b14160 ("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@brauner
Acked-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>

diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index 5ff001140ef4abe1437531ce85d2b8530d81aade..b7e3763c47f0cdf6d27b8145ffbb26b281179311 100644 (file)
--- a/drivers/mtd/mtdsuper.c
+++ b/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;