[PATCH v1 net] ipv6: Prevent rt6_insert_exception() for dying fib6_info.

Before the cited commit, fib6_nh_flush_exceptions() always set
from->exception_bucket_flushed = 1 under rt6_exception_lock to
prevent rt6_insert_exception() from inserting a new exception
for a dying fib6_info.

The flag was replaced with the FIB6_EXCEPTION_BUCKET_FLUSHED
bit stored in nh->rt6i_exception_bucket.

The problem is that now the bit is only set when the bucket
is not NULL and fib6_nh_flush_exceptions() is called from
fib6_nh_release() after fib6_ref has already reached zero.

If rt6_insert_exception() is called while the target fib6_info
is being removed via fib6_purge_rt(), a new exception could be
created successfully because rt6_flush_exceptions() no longer
sets the bit.

This creates a reference cycle between the fib6_info and the
exception route, leaking the fib6_info, its nexthop device,
and all per-CPU routes in fib6_nh->rt6i_pcpu, which stalls netdev
unregistration.

[   34.680602] unregister_netdevice: waiting for gre6 to become free. Usage count = 68
[   44.920675] unregister_netdevice: waiting for gre6 to become free. Usage count = 68
[   55.176582] unregister_netdevice: waiting for gre6 to become free. Usage count = 68

Let's call fib6_drop_pcpu_from() before rt6_flush_exceptions(),
to set fib6_destroying before rt6_exception_lock, and check
f6i->fib6_destroying in rt6_insert_exception().

Note that FIB6_EXCEPTION_BUCKET_FLUSHED logic is dead and
we can clean it up in net-next.

Fixes: cc5c073a693f ("ipv6: Move exception bucket to fib6_nh")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/ip6_fib.c | 2 +-
 net/ipv6/route.c   | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 9ea75703b38d..9ff761962b45 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c

@@ -1043,8 +1043,8 @@ static void fib6_purge_rt(struct fib6_info *rt, struct fib6_node *fn,
 	struct fib6_table *table = rt->fib6_table;
 
 	/* Flush all cached dst in exception table */
-	rt6_flush_exceptions(rt);
 	fib6_drop_pcpu_from(rt);
+	rt6_flush_exceptions(rt);
 
 	if (rt->nh) {
 		spin_lock(&rt->nh->lock);

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index b18cd0d9148c..5c898bfc4bd9 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c

@@ -1729,6 +1729,11 @@ static int rt6_insert_exception(struct rt6_info *nrt,
 
 	spin_lock_bh(&rt6_exception_lock);
 
+	if (f6i->fib6_destroying) {
+		err = -ENOENT;
+		goto out;
+	}
+
 	bucket = rcu_dereference_protected(nh->rt6i_exception_bucket,
 					  lockdep_is_held(&rt6_exception_lock));
 	if (!bucket) {

-- 
2.55.0.1082.g2b9226bbc0-goog