FAILED: patch "[PATCH] net: ipv6: ensure we call ipv6_mc_down() at most once" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9995b408f17ff8c7f11bc725c8aa225ba3a63b1c Mon Sep 17 00:00:00 2001
From: "[email protected]" <[email protected]>
Date: Thu, 24 Feb 2022 10:06:49 +0100
Subject: [PATCH] net: ipv6: ensure we call ipv6_mc_down() at most once

There are two reasons for addrconf_notify() to be called with NETDEV_DOWN:
either the network device is actually going down, or IPv6 was disabled on the interface.

If either of them stays down while the other is toggled, we repeatedly call the code for NETDEV_DOWN, including ipv6_mc_down(), while never calling the corresponding ipv6_mc_up() in between. This will cause a new entry in idev->mc_tomb to be allocated for each multicast group the interface is subscribed to, which in turn leaks one struct ifmcaddr6 per nontrivial multicast group the interface is subscribed to.

The following reproducer will leak at least $n objects:

ip addr add ff2e::4242/32 dev eth0 autojoin sysctl -w net.ipv6.conf.eth0.disable_ipv6=1 for i in $(seq 1 $n); do
ip link set up eth0; ip link set down eth0
done

Joining groups with IPV6_ADD_MEMBERSHIP (unprivileged) or setting the sysctl net.ipv6.conf.eth0.forwarding to 1 (=> subscribing to ff02::2) can also be used to create a nontrivial idev->mc_list, which will the leak objects with the right up-down-sequence.

Based on both sources for NETDEV_DOWN events the interface IPv6 state should be considered:

- not ready if the network interface is not ready OR IPv6 is disabled for it
- ready if the network interface is ready AND IPv6 is enabled for it

The functions ipv6_mc_up() and ipv6_down() should only be run when this state changes.

Implement this by remembering when the IPv6 state is ready, and only run ipv6_mc_down() if it actually changed from ready to not ready.

The other direction (not ready -> ready) already works correctly, as:

- the interface notification triggered codepath for NETDEV_UP / NETDEV_CHANGE returns early if ipv6 is disabled, and
- the disable_ipv6=0 triggered codepath skips fully initializing the interface as long as addrconf_link_ready(dev) returns false
- calling ipv6_mc_up() repeatedly does not leak anything

index 6c8ab3e6e6fe..f908e2fd30b2 100644
@@ -3732,6 +3732,7 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 	struct inet6_dev *idev;
 	struct inet6_ifaddr *ifa, *tmp;
 	bool keep_addr = false;
+	bool was_ready;
 	int state, i;
 
 	ASSERT_RTNL();

@@ -3797,7 +3798,10 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 
 	addrconf_del_rs_timer(idev);
 
-	/* Step 2: clear flags for stateless addrconf */
+	/* Step 2: clear flags for stateless addrconf, repeated down
+	 *         detection
+	 */
+	was_ready = idev->if_flags & IF_READY;
 	if (!unregister)
 		idev->if_flags &= ~(IF_RS_SENT|IF_RA_RCVD|IF_READY);
 

@@ -3871,7 +3875,7 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 	if (unregister) {
 		ipv6_ac_destroy_dev(idev);
 		ipv6_mc_destroy_dev(idev);
-	} else {
+	} else if (was_ready) {
 		ipv6_mc_down(idev);
 	}