netdev - Re: [PATCH net] ipvlan: add cond_resched_rcu() while processing muticast backlog

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Date:   Mon, 9 Mar 2020 18:07:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Mahesh Bandewar <maheshb@...gle.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Mahesh Bandewar <mahesh@...dewar.net>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipvlan: add cond_resched_rcu() while processing
 muticast backlog



On 3/9/20 3:57 PM, Mahesh Bandewar wrote:
> If there are substantial number of slaves created as simulated by
> Syzbot, the backlog processing could take much longer and result
> into the issue found in the Syzbot report.
> 

...

> 
> Fixes: ba35f8588f47 (“ipvlan: Defer multicast / broadcast processing to a work-queue”)
> Signed-off-by: Mahesh Bandewar <maheshb@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> ---
>  drivers/net/ipvlan/ipvlan_core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ipvlan/ipvlan_core.c b/drivers/net/ipvlan/ipvlan_core.c
> index 53dac397db37..5759e91dec71 100644
> --- a/drivers/net/ipvlan/ipvlan_core.c
> +++ b/drivers/net/ipvlan/ipvlan_core.c
> @@ -277,6 +277,7 @@ void ipvlan_process_multicast(struct work_struct *work)
>  			}
>  			ipvlan_count_rx(ipvlan, len, ret == NET_RX_SUCCESS, true);
>  			local_bh_enable();
> +			cond_resched_rcu();

This does not work : If you release rcu_read_lock() here,
then the surrounding loop can not be continued without risking use-after-free

rcu_read_lock();
list_for_each_entry_rcu(ipvlan, &port->ipvlans, pnode) {
    ...
    cond_resched_rcu();
    // after this point bad things can happen
}


You probably should do instead :

diff --git a/drivers/net/ipvlan/ipvlan_core.c b/drivers/net/ipvlan/ipvlan_core.c
index 30cd0c4f0be0b4d1dea2c0a4d68d0e33d1931ebc..57617ff5565fb87035c13dcf1de9fa5431d04e10 100644
--- a/drivers/net/ipvlan/ipvlan_core.c
+++ b/drivers/net/ipvlan/ipvlan_core.c
@@ -293,6 +293,7 @@ void ipvlan_process_multicast(struct work_struct *work)
                }
                if (dev)
                        dev_put(dev);
+               cond_resched();
        }
 }