Re: [PATCH] Fix use after free in get_tree_bdev()

[Lukas Czerner <lczerner@redhat.com>]

On Tue, Apr 28, 2020 at 09:27:48PM +0100, David Howells wrote:
> Commit 6fcf0c72e4b9, a fix to get_tree_bdev() put a missing blkdev_put() in
> the wrong place, before a warnf() that displays the bdev under
> consideration rather after it.
> 
> This results in a silent lockup in printk("%pg") called via warnf() from
> get_tree_bdev() under some circumstances when there's a race with the
> blockdev being frozen.  This can be caused by xfstests/tests/generic/085 in
> combination with Lukas Czerner's ext4 mount API conversion patchset.  It
> looks like it ought to occur with other users of get_tree_bdev() such as
> XFS, but apparently doesn't.
> 
> Fix this by switching the order of the lines.

This fixes the problem I was seeing. Thanks David.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>

> 
> Fixes: 6fcf0c72e4b9 ("vfs: add missing blkdev_put() in get_tree_bdev()")
> Reported-by: Lukas Czerner <lczerner@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Ian Kent <raven@themaw.net>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> ---
> 
>  fs/super.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index cd352530eca9..a288cd60d2ae 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1302,8 +1302,8 @@ int get_tree_bdev(struct fs_context *fc,
>  	mutex_lock(&bdev->bd_fsfreeze_mutex);
>  	if (bdev->bd_fsfreeze_count > 0) {
>  		mutex_unlock(&bdev->bd_fsfreeze_mutex);
> -		blkdev_put(bdev, mode);
>  		warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
> +		blkdev_put(bdev, mode);
>  		return -EBUSY;
>  	}
>  
> 
>