netdev - [PATCH net-next] ipv6: blackhole_netdev needs snmp6 counters

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Date:   Sun, 13 Feb 2022 18:10:56 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Ido Schimmel <idosch@...dia.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: [PATCH net-next] ipv6: blackhole_netdev needs snmp6 counters

From: Ido Schimmel <idosch@...dia.com>

Whenever rt6_uncached_list_flush_dev() swaps rt->rt6_idev
to the blackhole device, parts of IPv6 stack might still need
to increment one SNMP counter.

Root cause, patch from Ido, changelog from Eric :)

This bug suggests that we need to audit rt->rt6_idev usages
and make sure they are properly using RCU protection.

Fixes: e5f80fcf869a ("ipv6: give an IPv6 dev to blackhole_netdev")
Signed-off-by: Ido Schimmel <idosch@...dia.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: syzbot <syzkaller@...glegroups.com>
---
 net/ipv6/addrconf.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 02d31d4fcab3b3d529c4fe3260216ecee1108e82..57fbd6f03ff8d118e50d8aa6ea0ab938a1bb3cbc 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -400,16 +400,16 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 	/* We refer to the device */
 	dev_hold_track(dev, &ndev->dev_tracker, GFP_KERNEL);
 
-	if (dev != blackhole_netdev) {
-		if (snmp6_alloc_dev(ndev) < 0) {
-			netdev_dbg(dev, "%s: cannot allocate memory for statistics\n",
-				   __func__);
-			neigh_parms_release(&nd_tbl, ndev->nd_parms);
-			dev_put_track(dev, &ndev->dev_tracker);
-			kfree(ndev);
-			return ERR_PTR(err);
-		}
+	if (snmp6_alloc_dev(ndev) < 0) {
+		netdev_dbg(dev, "%s: cannot allocate memory for statistics\n",
+			   __func__);
+		neigh_parms_release(&nd_tbl, ndev->nd_parms);
+		dev_put_track(dev, &ndev->dev_tracker);
+		kfree(ndev);
+		return ERR_PTR(err);
+	}
 
+	if (dev != blackhole_netdev) {
 		if (snmp6_register_dev(ndev) < 0) {
 			netdev_dbg(dev, "%s: cannot create /proc/net/dev_snmp6/%s\n",
 				   __func__, dev->name);
-- 
2.35.1.265.g69c8d7142f-goog