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Date:   Thu,  7 May 2020 12:19:03 -0700
From:   Cong Wang <>
Cc:     Cong Wang <>,,,
        Jarod Wilson <>,
        Nikolay Aleksandrov <>,
        Josh Poimboeuf <>,
        Jann Horn <>,
        Jay Vosburgh <>
Subject: [Patch net v3] net: fix a potential recursive NETDEV_FEAT_CHANGE

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 when the LRO feature fails to change,
so it goes back and forth recursively until the kernel stack is

Commit 17b85d29e82c intentionally lets __netdev_update_features()
return -1 for such a failure case, so we have to just rely on
the existing check inside netdev_sync_lower_features() and skip
NETDEV_FEAT_CHANGE event only for this specific failure case.

Fixes: fd867d51f889 ("net/core: generic support for disabling netdev features down stack")
Cc: Jarod Wilson <>
Cc: Nikolay Aleksandrov <>
Cc: Josh Poimboeuf <>
Cc: Jann Horn <>
Reviewed-by: Jay Vosburgh <>
Signed-off-by: Cong Wang <>
 net/core/dev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 522288177bbd..6d327b7aa813 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8907,11 +8907,13 @@ 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",
 					    &feature, lower->name);
+			else
+				netdev_features_change(lower);

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