netdev - Re: [Patch net] vlan: fix a use-after-free in vlan_device

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Date:   Thu, 9 Nov 2017 17:46:15 -0800
From:   Girish Moodalbail <girish.moodalbail@...cle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     fengguang.wu@...el.com,
        Alexander Duyck <alexander.duyck@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Patch net] vlan: fix a use-after-free in vlan_device_event()

On 11/9/17 4:43 PM, Cong Wang wrote:
> After refcnt reaches zero, vlan_vid_del() could free
> dev->vlan_info via RCU:
> 
> 	RCU_INIT_POINTER(dev->vlan_info, NULL);
> 	call_rcu(&vlan_info->rcu, vlan_info_rcu_free);
> 
> However, the pointer 'grp' still points to that memory
> since it is set before vlan_vid_del():
> 
>          vlan_info = rtnl_dereference(dev->vlan_info);
>          if (!vlan_info)
>                  goto out;
>          grp = &vlan_info->grp;
> 
> Depends on when that RCU callback is scheduled, we could
> trigger a use-after-free in vlan_group_for_each_dev()
> right following this vlan_vid_del().
> 
> Fix it by moving vlan_vid_del() before setting grp. This
> is also symmetric to the vlan_vid_add() we call in
> vlan_device_event().
> 
> Reported-by: Fengguang Wu <fengguang.wu@...el.com>
> Fixes: efc73f4bbc23 ("net: Fix memory leak - vlan_info struct")
> Cc: Alexander Duyck <alexander.duyck@...il.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Girish Moodalbail <girish.moodalbail@...cle.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

LGTM.

Reviewed-by: Girish Moodalbail <girish.moodalbail@...cle.com>

Thanks,
~Girish


> ---
>   net/8021q/vlan.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 9649579b5b9f..4a72ee4e2ae9 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -376,6 +376,9 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>   			dev->name);
>   		vlan_vid_add(dev, htons(ETH_P_8021Q), 0);
>   	}
> +	if (event == NETDEV_DOWN &&
> +	    (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER))
> +		vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
>   
>   	vlan_info = rtnl_dereference(dev->vlan_info);
>   	if (!vlan_info)
> @@ -423,9 +426,6 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>   		struct net_device *tmp;
>   		LIST_HEAD(close_list);
>   
> -		if (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER)
> -			vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
> -
>   		/* Put all VLANs for this dev in the down state too.  */
>   		vlan_group_for_each_dev(grp, i, vlandev) {
>   			flgs = vlandev->flags;
>