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

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Message-ID: <769a5512.127aa.17867c56a27.Coremail.lyl2019@mail.ustc.edu.cn>
Date:   Thu, 25 Mar 2021 13:04:54 +0800 (GMT+08:00)
From:   lyl2019@...l.ustc.edu.cn
To:     "Vivek Goyal" <vgoyal@...hat.com>
Cc:     stefanha@...hat.com, miklos@...redi.hu,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] fuse: Fix a potential double free in
 virtio_fs_get_tree




> -----原始邮件-----
> 发件人: "Vivek Goyal" <vgoyal@...hat.com>
> 发送时间: 2021-03-24 01:10:03 (星期三)
> 收件人: "Lv Yunlong" <lyl2019@...l.ustc.edu.cn>
> 抄送: stefanha@...hat.com, miklos@...redi.hu, virtualization@...ts.linux-foundation.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
> 主题: 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
> > 
> > 
> 


Ok, thanks.
It should be a false positive.