[Patch net] net: fix a potential recursive NETDEV_FEAT_CHANGE

[Cong Wang <xiyou.wangcong@gmail.com>]

syzbot managed to trigger a recursive NETDEV_FEAT_CHANGE event
between bonding master and slave. I managed to find a reproducer
for this:

  ip li set bond0 up
  ifenslave bond0 eth0
  brctl addbr br0
  ethtool -K eth0 lro off
  brctl addif br0 bond0
  ip li set br0 up

When a NETDEV_FEAT_CHANGE event is triggered on a bonding slave,
it captures this and calls bond_compute_features() to fixup its
master's and other slaves' features. However, when syncing with
its lower devices by netdev_sync_lower_features() this event is
triggered again on slaves, so it goes back and forth recursively
until the kernel stack is exhausted.

It is unnecessary to trigger it for a second time, because when
we update the features from top down, we rely on each
dev->netdev_ops->ndo_fix_features() to do the job, each stacked
device should implement it. NETDEV_FEAT_CHANGE event is necessary
when we update from bottom up, like in existing stacked device
implementations.

Just calling __netdev_update_features() is sufficient to fix this
issue.

Fixes: fd867d51f889 ("net/core: generic support for disabling netdev features down stack")
Reported-by: syzbot+e73ceacfd8560cc8a3ca@syzkaller.appspotmail.com
Reported-by: syzbot+c2fb6f9ddcea95ba49b5@syzkaller.appspotmail.com
Cc: Jarod Wilson <jarod@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jay Vosburgh <j.vosburgh@gmail.com>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 522288177bbd..ece50ae346c3 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8907,7 +8907,7 @@ static void netdev_sync_lower_features(struct net_device *upper,
 			netdev_dbg(upper, "Disabling feature %pNF on lower dev %s.\n",
 				   &feature, lower->name);
 			lower->wanted_features &= ~feature;
-			netdev_update_features(lower);
+			__netdev_update_features(lower);
 
 			if (unlikely(lower->features & feature))
 				netdev_WARN(upper, "failed to disable %pNF on %s!\n",
-- 
2.26.2