netdev - Re: [PATCH net v2] ipv6: Handle race in addrconf_dad

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Message-ID: <4c896029-882f-1a4f-c0cc-4553a9429da3@gmail.com>
Date:   Thu, 3 Oct 2019 09:50:13 -0600
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsahern@...nel.org>, davem@...emloft.net,
        jakub.kicinski@...ronome.com
Cc:     netdev@...r.kernel.org, rajendra.dendukuri@...adcom.com
Subject: Re: [PATCH net v2] ipv6: Handle race in addrconf_dad_work

On 10/2/19 3:23 PM, Eric Dumazet wrote:
> 
> 
> On 10/2/19 2:08 PM, Eric Dumazet wrote:
>>
>>
>> On 10/1/19 11:18 AM, Eric Dumazet wrote:
>>>
>>>
>>> On 9/30/19 8:28 PM, David Ahern wrote:
>>>> From: David Ahern <dsahern@...il.com>
>>>>
>>>> Rajendra reported a kernel panic when a link was taken down:
>>>>
>>>> [ 6870.263084] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a8
>>>> [ 6870.271856] IP: [<ffffffff8efc5764>] __ipv6_ifa_notify+0x154/0x290
>>>>
>>>> <snip>
>>>>
>>>
>>> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
>>>
>>> Thanks !
>>>
>>
>> Apparently this patch causes problems. I yet have to make an analysis.
>>
> 
> It seems we need to allow the code to do some changes if IF_READY is not set.
> 

That statement was correct. Prior to the patch in question ifp->state is
bumped to INET6_IFADDR_STATE_DAD in addrconf_dad_work. When
NETDEV_CHANGE event happens, addrconf_notify calls addrconf_dad_run
which calls addrconf_dad_kick to restart dad after the dad process
(applies even if nodad is set -- yes, odd).

With the patch, IF_READY is not set when the bond device is created and
dad_work skips bumping the state. When the CHANGE event comes through
the state is not INET6_IFADDR_STATE_DAD (and the restart argument is not
set), so addrconf_dad_run does not call addrconf_dad_kick.

Bottom line, regardless of IF_READY we need the state change to happen
in dad_work, we just need to skip the call to addconf_dad_begin.

Can you test the change below on your boxes? It applies on current net.

Rajendra: can you test as well and make sure your problem is still
resolved?

Thanks,

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index dd3be06d5a06..fce0d0dca7bb 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -4032,12 +4032,6 @@ static void addrconf_dad_work(struct work_struct *w)

        rtnl_lock();

-       /* check if device was taken down before this delayed work
-        * function could be canceled
-        */
-       if (idev->dead || !(idev->if_flags & IF_READY))
-               goto out;
-
        spin_lock_bh(&ifp->lock);
        if (ifp->state == INET6_IFADDR_STATE_PREDAD) {
                action = DAD_BEGIN;
@@ -4068,6 +4062,12 @@ static void addrconf_dad_work(struct work_struct *w)
        }
        spin_unlock_bh(&ifp->lock);

+       /* check if device was taken down before this delayed work
+        * function could be canceled
+        */
+       if (idev->dead || !(idev->if_flags & IF_READY))
+               goto out;
+
        if (action == DAD_BEGIN) {
                addrconf_dad_begin(ifp);
                goto out;