Re: Re: [Drbd-dev] [PATCH] drbd: Fix a use after free in get_initial_state

[lyl2019@mail.ustc.edu.cn]

> -----原始邮件-----
> 发件人: "Christoph Böhmwalder" <christoph.boehmwalder@linbit.com>
> 发送时间: 2021-04-01 21:01:20 (星期四)
> 收件人: "Lv Yunlong" <lyl2019@mail.ustc.edu.cn>
> 抄送: philipp.reisner@linbit.com, lars.ellenberg@linbit.com, axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, drbd-dev@lists.linbit.com
> 主题: Re: [Drbd-dev] [PATCH] drbd: Fix a use after free in get_initial_state
> 
> On 4/1/21 1:57 PM, Lv Yunlong wrote:
> > In get_initial_state, it calls notify_initial_state_done(skb,..) if
> > cb->args[5]==1. I see that if genlmsg_put() failed in
> > notify_initial_state_done(), the skb will be freed by nlmsg_free(skb).
> > Then get_initial_state will goto out and the freed skb will be used by
> > return value skb->len.
> > 
> > My patch lets skb_len = skb->len and return the skb_len to avoid the uaf.
> > 
> > Fixes: a29728463b254 ("drbd: Backport the "events2" command")
> > Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> > ---
> >   drivers/block/drbd/drbd_nl.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
> > index bf7de4c7b96c..474f84675d0a 100644
> > --- a/drivers/block/drbd/drbd_nl.c
> > +++ b/drivers/block/drbd/drbd_nl.c
> > @@ -4905,6 +4905,7 @@ static int get_initial_state(struct sk_buff *skb, struct netlink_callback *cb)
> >   	struct drbd_state_change *state_change = (struct drbd_state_change *)cb->args[0];
> >   	unsigned int seq = cb->args[2];
> >   	unsigned int n;
> > +	unsigned int skb_len = skb->len;
> >   	enum drbd_notification_type flags = 0;
> >   
> >   	/* There is no need for taking notification_mutex here: it doesn't
> > @@ -4915,7 +4916,7 @@ static int get_initial_state(struct sk_buff *skb, struct netlink_callback *cb)
> >   	cb->args[5]--;
> >   	if (cb->args[5] == 1) {
> >   		notify_initial_state_done(skb, seq);
> > -		goto out;
> > +		return skb_len;
> >   	}
> >   	n = cb->args[4]++;
> >   	if (cb->args[4] < cb->args[3])
> > 
> 
> Thanks for the patch!
> 
> I think the problem goes even further: skb can also be freed in the 
> notify_*_state_change -> notify_*_state calls below.
> 
> Also, at the point where we save skb->len into skb_len, skb is not 
> initialized yet. Maybe it makes more sense to not return a length in the 
> first place here, but an error code instead.
> 
> -- 
> Christoph Böhmwalder
> LINBIT | Keeping the Digital World Running
> DRBD HA —  Disaster Recovery — Software defined Storage

Ok, I see.
I found that drbd_adm_get_initial_state() has called the get_initial_state(),
and return -ENOMEM if it calls remember_old_state() failed.

So, i think that means if get_initial_state() failed on the notify_initial_state_done(),
it should return -ENOMEM too.

I will submit the PATCH v2 to fix the first place. The fixes of the further problem is 
hard for me.

Thanks.