[PATCH net] net: ipv6: regenerate host route if moved to gc list

[David Ahern <dsa@cumulusnetworks.com>]

Taking down the loopback device wreaks havoc on IPv6 routes. By
extension, taking a VRF device wreaks havoc on its table.

Dmitry and Andrey both reported heap out-of-bounds reports in the IPv6
FIB code while running syzkaller fuzzer. The root cause is a dead dst
that is on the garbage list gets reinserted into the IPv6 FIB. While on
the gc (or perhaps when it gets added to the gc list) the dst->next is
set to an IPv4 dst. A subsequent walk of the ipv6 tables causes the
out-of-bounds access.

Andrey's reproducer was the key to getting to the bottom of this.

With IPv6, host routes for an address have the dst->dev set to the
loopback device. When the 'lo' device is taken down, rt6_ifdown initiates
a walk of the fib evicting routes with the 'lo' device which means all
host routes are removed. That process moves the dst which is attached to
an inet6_ifaddr to the gc list and marks it as dead.

The recent change to keep global IPv6 addresses added a new function
fixup_permanent_addr that is called on admin up. That function restarts
dad for an inet6_ifaddr and when it completes the host route attached
to it is inserted into the fib. Since the route was marked dead and
moved to the gc list, we get the reported out-of-bounds accesses. If
the device with the address is taken down or the address is removed, the
WARN_ON in fib6_del is triggered.

All of those faults are fixed by regenerating the host route of the
existing one has been moved to the gc list, something that can be
determined by checking if the rt6i_ref counter is 0.

The update of the route on the ifp is done using cmpxchg as suggested
by Li RongQing in a patch a year ago.

Fixes: f1705ec197e7 ("net: ipv6: Make address flushing on ifdown optional")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
---
 net/ipv6/addrconf.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 08f9e8ea7a81..93555528a45b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3303,14 +3303,16 @@ static void addrconf_gre_config(struct net_device *dev)
 static int fixup_permanent_addr(struct inet6_dev *idev,
 				struct inet6_ifaddr *ifp)
 {
-	if (!ifp->rt) {
-		struct rt6_info *rt;
+	if (!ifp->rt || !atomic_read(&ifp->rt->rt6i_ref)) {
+		struct rt6_info *rt, *prev;
 
 		rt = addrconf_dst_alloc(idev, &ifp->addr, false);
 		if (unlikely(IS_ERR(rt)))
 			return PTR_ERR(rt);
 
-		ifp->rt = rt;
+		prev = cmpxchg(&ifp->rt, ifp->rt, rt);
+		if (prev)
+			ip6_rt_put(prev);
 	}
 
 	if (!(ifp->flags & IFA_F_NOPREFIXROUTE)) {
-- 
2.1.4