linux-kernel - Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get

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Date:   Tue, 23 Mar 2021 13:10:03 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Lv Yunlong <lyl2019@...l.ustc.edu.cn>
Cc:     stefanha@...hat.com, miklos@...redi.hu,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fuse: Fix a potential double free in virtio_fs_get_tree

On Mon, Mar 22, 2021 at 10:18:31PM -0700, Lv Yunlong wrote:
> In virtio_fs_get_tree, fm is allocated by kzalloc() and
> assigned to fsc->s_fs_info by fsc->s_fs_info=fm statement.
> If the kzalloc() failed, it will goto err directly, so that
> fsc->s_fs_info must be non-NULL and fm will be freed.

sget_fc() will either consume fsc->s_fs_info in case a new super
block is allocated and set fsc->s_fs_info. In that case we don't
free fc or fm.

Or, sget_fc() will return with fsc->s_fs_info set in case we already
found a super block. In that case we need to free fc and fm.

In case of error from sget_fc(), fc/fm need to be freed first and
then error needs to be returned to caller.

        if (IS_ERR(sb))
                return PTR_ERR(sb);


If we allocated a new super block in sget_fc(), then next step is
to initialize it.

        if (!sb->s_root) {
                err = virtio_fs_fill_super(sb, fsc);
	}

If we run into errors here, then fc/fm need to be freed.

So current code looks fine to me.

Vivek

> 
> But later fm is freed again when virtio_fs_fill_super() fialed.
> I think the statement if (fsc->s_fs_info) {kfree(fm);} is
> misplaced.
> 
> My patch puts this statement in the correct palce to avoid
> double free.
> 
> Signed-off-by: Lv Yunlong <lyl2019@...l.ustc.edu.cn>
> ---
>  fs/fuse/virtio_fs.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 8868ac31a3c0..727cf436828f 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1437,10 +1437,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  
>  	fsc->s_fs_info = fm;
>  	sb = sget_fc(fsc, virtio_fs_test_super, set_anon_super_fc);
> -	if (fsc->s_fs_info) {
> -		fuse_conn_put(fc);
> -		kfree(fm);
> -	}
> +
>  	if (IS_ERR(sb))
>  		return PTR_ERR(sb);
>  
> @@ -1457,6 +1454,11 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  		sb->s_flags |= SB_ACTIVE;
>  	}
>  
> +	if (fsc->s_fs_info) {
> +		fuse_conn_put(fc);
> +		kfree(fm);
> +	}
> +
>  	WARN_ON(fsc->root);
>  	fsc->root = dget(sb->s_root);
>  	return 0;
> -- 
> 2.25.1
> 
>