[PATCH 3.16 180/204] vlan: fix a use-after-free in vlan_device_event()

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Cong Wang <xiyou.wangcong@xxxxxxxxx>

commit 052d41c01b3a2e3371d66de569717353af489d63 upstream.

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@xxxxxxxxx>
Fixes: efc73f4bbc23 ("net: Fix memory leak - vlan_info struct")
Cc: Alexander Duyck <alexander.duyck@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Girish Moodalbail <girish.moodalbail@xxxxxxxxxx>
Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Reviewed-by: Girish Moodalbail <girish.moodalbail@xxxxxxxxxx>
Tested-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/8021q/vlan.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -364,6 +364,9 @@ static int vlan_device_event(struct noti
 			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)
@@ -408,9 +411,6 @@ static int vlan_device_event(struct noti
 		break;
 
 	case NETDEV_DOWN:
-		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;