Subject: Re: [Patch net] net: fix a potential recursive NETDEV_FEAT_CHANGE

From: Jay Vosburgh <[email-masked]>
Date: Tue, 05 May 2020 15:39:57 -0700

Cong Wang <[email-masked]> wrote:

>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

	Presumably this is tied to the LRO feature being special in
netdev_sync_lower_features (via NETIF_F_UPPER_DISABLES), but why doesn't
LRO become disabled and stop the recursion once the test

		if (!(features & feature) && (lower->features & feature)) {

	no longer evalutes to true (in theory)?

	-J

>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: [email-masked]
>Reported-by: [email-masked]
>Cc: Jarod Wilson <[email-masked]>
>Cc: Josh Poimboeuf <[email-masked]>
>Cc: Jay Vosburgh <[email-masked]>
>Cc: Jann Horn <[email-masked]>
>Signed-off-by: Cong Wang <[email-masked]>
>---
> 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
>

---
	-Jay Vosburgh, [email-masked]