linux-kernel - Re: [PATCH vfs/for-next] vfs: fix vfs_get_single_reconf

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Date:   Thu, 5 Sep 2019 20:01:39 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot+5aca688dac0796c56129@...kaller.appspotmail.com
Subject: Re: [PATCH vfs/for-next] vfs: fix vfs_get_single_reconf_super error
 handling

On Fri, Aug 30, 2019 at 10:10:24PM -0500, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> syzbot reported an invalid free in debugfs_release_dentry().  The
> reproducer tries to mount debugfs with the 'dirsync' option, which is
> not allowed.  The bug is that if reconfigure_super() fails in
> vfs_get_super(), deactivate_locked_super() is called, but also
> fs_context::root is left non-NULL which causes deactivate_super() to be
> called again later.
> 
> Fix it by releasing fs_context::root in the error path.
> 
> Reported-by: syzbot+5aca688dac0796c56129@...kaller.appspotmail.com
> Fixes: e478b48498a7 ("vfs: Add a single-or-reconfig keying to vfs_get_super()")
> Cc: David Howells <dhowells@...hat.com>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  fs/super.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 0f913376fc4c..99195e15be05 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1194,8 +1194,11 @@ int vfs_get_super(struct fs_context *fc,
>  		fc->root = dget(sb->s_root);
>  		if (keying == vfs_get_single_reconf_super) {
>  			err = reconfigure_super(fc);
> -			if (err < 0)
> +			if (err < 0) {
> +				dput(fc->root);
> +				fc->root = NULL;
>  				goto error;
> +			}
>  		}
>  	}
>  

Ping.  This is still broken in linux-next.

- Eric