[PATCH net-next v3 2/4] ipv6: take fib6_sernum from a sequence shared by all netns

[Qiliang Yuan <qiliang.yuan@linux.dev>]

A per-cpu IPv6 route of a nexthop object records fib6_sernum of the
netns it was created in, and rt6_is_valid() compares it with fib6_sernum
of the netns its device belongs to when the route is checked. Every netns
starts its fib6_sernum at 1 and increments it on each tree change, so
different netns often hold the same value.

When a device moves to another netns, such a route remains usable until
nexthop_free_rcu() releases it, and fib6_info_destroy_rcu() queues that
callback from inside an RCU callback, after the device has moved. If the
fib6_sernum of the new netns matches the cached one in that window,
ip6_dst_check() accepts the route and a socket in the old netns keeps
sending through a device that now belongs to another netns.

Take every fib6_sernum bump from one sequence shared by all netns, which
only hands out values from 2 up: every netns still starts at 1, and 0
means there is no sernum to check. Inserting a route bumps fib6_sernum,
so the routes a netns caches carry values drawn from the sequence, which
no other netns holds until it wraps after 2^31 bumps. fib6_new_sernum()
callers only hold their own table's tb6_lock, so two tables of one netns
can store their values in either order, and fib6_sernum is no longer
monotonic per netns. It is only compared for equality, so this costs at
most an extra invalidation. A connected UDP socket in netns A caches a
route through an IPv6 nexthop object on a veth. The veth is moved to
netns B and brought up with one RTM_NEWLINK, then the socket sends
200000 datagrams. A route is added to and removed from B 0 to 20 times
before the move, 63 trials:

                                before     after
  trials sending after move          1         0
  datagrams sent after move       3429         0

Fixes: 8f34e53b60b3 ("ipv6: Use global sernum for dst validation with nexthop objects")
Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/ip6_fib.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index e1d49897d6ab0..827051cbf7482 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -123,13 +123,23 @@ static void fib6_walkers_skip_rt(struct net *net, struct fib6_info *rt)
 	fib6_walkers_replace_leaf(net, rt, next);
 }
 
+/*
+ * Draw fib6_sernum bumps from one sequence shared by all netns, so that a
+ * route cached against one netns does not match the sernum of another one,
+ * e.g. after its device moved, until the sequence wraps. Every netns starts
+ * at 1 and the sequence only hands out values from 2 up: 0 means no sernum
+ * to check.
+ */
+static atomic_t fib6_sernum_seq = ATOMIC_INIT(1);
+
 static int fib6_new_sernum(struct net *net)
 {
-	int new, old = atomic_read(&net->ipv6.fib6_sernum);
+	int new, old = atomic_read(&fib6_sernum_seq);
 
 	do {
-		new = old < INT_MAX ? old + 1 : 1;
-	} while (!atomic_try_cmpxchg(&net->ipv6.fib6_sernum, &old, new));
+		new = old < INT_MAX ? old + 1 : 2;
+	} while (!atomic_try_cmpxchg(&fib6_sernum_seq, &old, new));
+	atomic_set(&net->ipv6.fib6_sernum, new);
 
 	return new;
 }

-- 
2.43.0