block: Initialize bd_bdi on inode initialization

So far we initialized bd_bdi only in bdget(). That is fine for normal
bdev inodes however for the special case of the root inode of
blockdev_superblock that function is never called and thus bd_bdi is
left uninitialized. As a result bdev_evict_inode() may oops doing
bdi_put(root->bd_bdi) on that inode as can be seen when doing:

mount -t bdev none /mnt

Fix the problem by initializing bd_bdi when first allocating the inode
and then reinitializing bd_bdi in bdev_evict_inode().

Thanks to syzkaller team for finding the problem.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: b1d2dc5659b4 ("block: Make blk_get_backing_dev_info() safe without open bdev")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 77c30f15a02c..2eca00ec4370 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -870,6 +870,7 @@ static void init_once(void *foo)
 #ifdef CONFIG_SYSFS
         INIT_LIST_HEAD(&bdev->bd_holder_disks);
 #endif
+        bdev->bd_bdi = &noop_backing_dev_info;
         inode_init_once(&ei->vfs_inode);
         /* Initialize mutex for freeze. */
         mutex_init(&bdev->bd_fsfreeze_mutex);
@@ -884,8 +885,10 @@ static void bdev_evict_inode(struct inode *inode)
         spin_lock(&bdev_lock);
         list_del_init(&bdev->bd_list);
         spin_unlock(&bdev_lock);
-        if (bdev->bd_bdi != &noop_backing_dev_info)
+        if (bdev->bd_bdi != &noop_backing_dev_info) {
                 bdi_put(bdev->bd_bdi);
+                bdev->bd_bdi = &noop_backing_dev_info;
+        }
 }
 
 static const struct super_operations bdev_sops = {
@@ -988,7 +991,6 @@ struct block_device *bdget(dev_t dev)
                 bdev->bd_contains = NULL;
                 bdev->bd_super = NULL;
                 bdev->bd_inode = inode;
-                bdev->bd_bdi = &noop_backing_dev_info;
                 bdev->bd_block_size = i_blocksize(inode);
                 bdev->bd_part_count = 0;
                 bdev->bd_invalidated = 0;