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Subject: Re: [Drbd-dev] [PATCH] drbd: Fix a use after free in
 get_initial_state
To:     Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Cc:     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
References: <20210401115753.3684-1-lyl2019@mail.ustc.edu.cn>
From:   =?UTF-8?Q?Christoph_B=c3=b6hmwalder?= 
        <christoph.boehmwalder@linbit.com>
Message-ID: <cb0f43e4-bfde-ac77-5153-f2f3cbed0172@linbit.com>
Date:   Thu, 1 Apr 2021 15:01:20 +0200
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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