linux-kernel - Re: [PATCH] fuse: Avoid potential use after free

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Message-ID: <20210407155031.GA1014852@redhat.com>
Date:   Wed, 7 Apr 2021 11:50:31 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Aditya Pakki <pakki001@....edu>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fuse: Avoid potential use after free

On Tue, Apr 06, 2021 at 06:53:32PM -0500, Aditya Pakki wrote:
> In virtio_fs_get_tree, after fm is freed, it is again freed in case
> s_root is NULL and virtio_fs_fill_super() returns an error. To avoid
> a double free, set fm to NULL.
> 
> Signed-off-by: Aditya Pakki <pakki001@....edu>
> ---
>  fs/fuse/virtio_fs.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 4ee6f734ba83..a7484c1539bf 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1447,6 +1447,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  	if (fsc->s_fs_info) {
>  		fuse_conn_put(fc);
>  		kfree(fm);
> +		fm = NULL;

I think both the code paths are mutually exclusive and that's why we
don't double free it.

sget_fc(), can either return existing super block which is already
initialized, or it can create a new super block which need to
initialize further.

If if get an existing super block, in that case fs->s_fs_info will
still be set and we need to free fm (as we did not use it). But in 
that case this super block is already initialized so sb->s_root
should be non-null and we will not call virtio_fs_fill_super()
on this. And hence we will not get into kfree(fm) again.

Same applies to fuse_conn_put(fc) call as well.

So I think this patch is not needed. I think sget_fc() semantics are
not obvious and that confuses the reader of the code.

Thanks
Vivek

>  	}
>  	if (IS_ERR(sb))
>  		return PTR_ERR(sb);
> -- 
> 2.25.1
>