Subject: CVE-2022-48910: net: ipv6: ensure we call ipv6_mc_down() at most once

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

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

The Linux kernel CVE team has assigned CVE-2022-48910 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 4.9.313 with commit 9a8736b2da28
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 4.14.278 with commit c71bf3229f9e
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 5.4.193 with commit 9588ac2eddc2
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 5.10.104 with commit f4c63b24dea9
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 5.15.27 with commit b11781515208
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 5.16.13 with commit 72124e65a70b
	Issue introduced in 3.18 with commit 3ce62a84d53c and fixed in 5.17 with commit 9995b408f17f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2022-48910
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/ipv6/addrconf.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/9a8736b2da28b24f01707f592ff059b9f90a058c
	https://git.kernel.org/stable/c/c71bf3229f9e9dd60ba02f5a5be02066edf57012
	https://git.kernel.org/stable/c/9588ac2eddc2f223ebcebf6e9f5caed84d32922b
	https://git.kernel.org/stable/c/f4c63b24dea9cc2043ff845dcca9aaf8109ea38a
	https://git.kernel.org/stable/c/b11781515208dd31fbcd0b664078dce5dc44523f
	https://git.kernel.org/stable/c/72124e65a70b84e6303a5cd21b0ac1f27d7d61a4
	https://git.kernel.org/stable/c/9995b408f17ff8c7f11bc725c8aa225ba3a63b1c