ipv6: mcast: Check inet6_dev->dead under idev->mc_lock in __ipv6_dev_mc_inc().

[ Upstream commit dbd40f318cf2f59759bd170c401adc20ba360a3e ]

Since commit 63ed8de4be81 ("mld: add mc_lock for protecting
per-interface mld data"), every multicast resource is protected
by inet6_dev->mc_lock.

RTNL is unnecessary in terms of protection but still needed for
synchronisation between addrconf_ifdown() and __ipv6_dev_mc_inc().

Once we removed RTNL, there would be a race below, where we could
add a multicast address to a dead inet6_dev.

  CPU1                            CPU2
  ====                            ====
  addrconf_ifdown()               __ipv6_dev_mc_inc()
                                    if (idev->dead) <-- false
    dead = true                       return -ENODEV;
    ipv6_mc_destroy_dev() / ipv6_mc_down()
      mutex_lock(&idev->mc_lock)
      ...
      mutex_unlock(&idev->mc_lock)
                                    mutex_lock(&idev->mc_lock)
                                    ...
                                    mutex_unlock(&idev->mc_lock)

The race window can be easily closed by checking inet6_dev->dead
under inet6_dev->mc_lock in __ipv6_dev_mc_inc() as addrconf_ifdown()
will acquire it after marking inet6_dev dead.

Let's check inet6_dev->dead under mc_lock in __ipv6_dev_mc_inc().

Note that now __ipv6_dev_mc_inc() no longer depends on RTNL and
we can remove ASSERT_RTNL() there and the RTNL comment above
addrconf_join_solict().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-4-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

2 files changed, 8 insertions, 10 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index f6188bd9f55b..1c3b0ba289fb 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2193,13 +2193,12 @@ errdad:
 	in6_ifa_put(ifp);
 }
 
-/* Join to solicited addr multicast group.
- * caller must hold RTNL */
+/* Join to solicited addr multicast group. */
 void addrconf_join_solict(struct net_device *dev, const struct in6_addr *addr)
 {
 	struct in6_addr maddr;
 
-	if (dev->flags&(IFF_LOOPBACK|IFF_NOARP))
+	if (READ_ONCE(dev->flags) & (IFF_LOOPBACK | IFF_NOARP))
 		return;
 
 	addrconf_addr_solict_mult(addr, &maddr);
@@ -3834,7 +3833,7 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 	 *	   Do not dev_put!
 	 */
 	if (unregister) {
-		idev->dead = 1;
+		WRITE_ONCE(idev->dead, 1);
 
 		/* protected by rtnl_lock */
 		RCU_INIT_POINTER(dev->ip6_ptr, NULL);
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index e153dac47a53..160b452f75e7 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -906,23 +906,22 @@ static struct ifmcaddr6 *mca_alloc(struct inet6_dev *idev,
 static int __ipv6_dev_mc_inc(struct net_device *dev,
 			     const struct in6_addr *addr, unsigned int mode)
 {
-	struct ifmcaddr6 *mc;
 	struct inet6_dev *idev;
-
-	ASSERT_RTNL();
+	struct ifmcaddr6 *mc;
 
 	/* we need to take a reference on idev */
 	idev = in6_dev_get(dev);
-
 	if (!idev)
 		return -EINVAL;
 
-	if (idev->dead) {
+	mutex_lock(&idev->mc_lock);
+
+	if (READ_ONCE(idev->dead)) {
+		mutex_unlock(&idev->mc_lock);
 		in6_dev_put(idev);
 		return -ENODEV;
 	}
 
-	mutex_lock(&idev->mc_lock);
 	for_each_mc_mclock(idev, mc) {
 		if (ipv6_addr_equal(&mc->mca_addr, addr)) {
 			mc->mca_users++;