Messages in this thread Patch in this message | | | Date | Fri, 29 Mar 2013 10:48:56 +0100 | From | Jiri Pirko <> | Subject | Re: [BUG] Crash with NULL pointer dereference in bond_handle_frame in -rt (possibly mainline) |
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Thu, Mar 28, 2013 at 06:29:52PM CET, eric.dumazet@gmail.com wrote: >On Thu, 2013-03-28 at 13:16 -0400, Steven Rostedt wrote: >> Hi, >> >> I'm currently debugging a crash in an old 3.0-rt kernel that one of our >> customers is seeing. The bug happens with a stress test that loads and >> unloads the bonding module in a loop (I don't know all the details as >> I'm not the one that is directly interacting with the customer). But the >> bug looks to be something that may still be present and possibly present >> in mainline too. It will just be much harder to trigger it in mainline. >> >> In -rt, interrupts are threads, and can schedule in and out just like >> any other thread. Note, mainline now supports interrupt threads so this >> may be easily reproducible in mainline as well. I don't have the ability >> to tell the customer to try mainline or other kernels, so my hands are >> somewhat tied to what I can do. >> >> But according to a core dump, I tracked down that the eth irq thread >> crashed in bond_handle_frame() here: >> >> slave = bond_slave_get_rcu(skb->dev); >> bond = slave->bond; <--- BUG >> >> >> the slave returned was NULL and accessing slave->bond caused a NULL >> pointer dereference. >> >> Looking at the code that unregisters the handler: >> >> void netdev_rx_handler_unregister(struct net_device *dev) >> { >> >> ASSERT_RTNL(); >> RCU_INIT_POINTER(dev->rx_handler, NULL); >> RCU_INIT_POINTER(dev->rx_handler_data, NULL); >> } >> >> Which is basically: >> dev->rx_handler = NULL; >> dev->rx_handler_data = NULL; >> >> And looking at __netif_receive_skb() we have: >> >> rx_handler = rcu_dereference(skb->dev->rx_handler); >> if (rx_handler) { >> if (pt_prev) { >> ret = deliver_skb(skb, pt_prev, orig_dev); >> pt_prev = NULL; >> } >> switch (rx_handler(&skb)) { >> >> My question to all of you is, what stops this interrupt from happening >> while the bonding module is unloading? What happens if the interrupt >> triggers and we have this: >> >> >> CPU0 CPU1 >> ---- ---- >> rx_handler = skb->dev->rx_handler >> >> netdev_rx_handler_unregister() { >> dev->rx_handler = NULL; >> dev->rx_handler_data = NULL; >> >> rx_handler() >> bond_handle_frame() { >> slave = skb->dev->rx_handler; >> bond = slave->bond; <-- NULL pointer dereference!!! >> >> >> What protection am I missing in the bond release handler that would >> prevent the above from happening?
Hmm. I think that this might be issue introduced by: commit a9b3cd7f323b2e57593e7215362a7b02fc933e3a Author: Stephen Hemminger <shemminger@vyatta.com> Date: Mon Aug 1 16:19:00 2011 +0000
rcu: convert uses of rcu_assign_pointer(x, NULL) to RCU_INIT_POINTER
Because, if rcu_dereference(dev->rx_handler) is null, rcu_dereference(dev->rx_handler_data) is never done. Therefore I believe we are hitting following scenario:
CPU0 CPU1 ---- ---- dev->rx_handler_data = NULL rcu_read_lock() dev->rx_handler = NULL
CPU0 will see rx_handler set and yet, rx_handler_data nulled. Write barrier in rcu_assign_pointer() might prevent this reorder from happening. Therefore I suggest:
diff --git a/net/core/dev.c b/net/core/dev.c index 0caa38e..c16b829 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3332,8 +3332,8 @@ void netdev_rx_handler_unregister(struct net_device *dev) { ASSERT_RTNL(); - RCU_INIT_POINTER(dev->rx_handler, NULL); - RCU_INIT_POINTER(dev->rx_handler_data, NULL); + rcu_assign_pointer(dev->rx_handler, NULL); + rcu_assign_pointer(dev->rx_handler_data, NULL); } EXPORT_SYMBOL_GPL(netdev_rx_handler_unregister); > >Nothing :( > >bug introduced in commit 35d48903e9781975e823b359ee85c257c9ff5c1c >(bonding: fix rx_handler locking) > >CC Jiri > >Fix seems simple : > >diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c >index 6bbd90e..7956ca5 100644 >--- a/drivers/net/bonding/bond_main.c >+++ b/drivers/net/bonding/bond_main.c >@@ -1457,6 +1457,8 @@ static rx_handler_result_t bond_handle_frame(struct sk_buff **pskb) > *pskb = skb; > > slave = bond_slave_get_rcu(skb->dev); >+ if (!slave) >+ return ret; > bond = slave->bond; > > if (bond->params.arp_interval) > > >
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