netdev - Re: [PATCH net] net: ipv6: regenerate host route if moved to gc list

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Message-ID: <CACT4Y+aRBZApshv2T-edaErPmiCsbMxNNkzC5Hs5jGJdvVwOAg@mail.gmail.com>
Date:   Sat, 22 Apr 2017 11:14:08 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     David Ahern <dsa@...ulusnetworks.com>,
        netdev <netdev@...r.kernel.org>,
        andreyknvl <andreyknvl@...gle.com>, mmanning@...cade.com
Subject: Re: [PATCH net] net: ipv6: regenerate host route if moved to gc list

On Sat, Apr 22, 2017 at 7:57 AM, Martin KaFai Lau <kafai@...com> wrote:
> On Fri, Apr 21, 2017 at 04:40:30PM -0700, David Ahern wrote:
>> Taking down the loopback device wreaks havoc on IPv6 routes. By
>> extension, taking a VRF device wreaks havoc on its table.
>>
>> Dmitry and Andrey both reported heap out-of-bounds reports in the IPv6
>> FIB code while running syzkaller fuzzer. The root cause is a dead dst
>> that is on the garbage list gets reinserted into the IPv6 FIB. While on
>> the gc (or perhaps when it gets added to the gc list) the dst->next is
>> set to an IPv4 dst. A subsequent walk of the ipv6 tables causes the
>> out-of-bounds access.
> Thanks for the investigation and details explanation.
>
> It sounds like the dst is already in DST_OBSOLETE_DEAD during
> the second fib6_add().  Glad that the fib6_del() caught it.
>
>>
>> Andrey's reproducer was the key to getting to the bottom of this.
>>
>> With IPv6, host routes for an address have the dst->dev set to the
>> loopback device. When the 'lo' device is taken down, rt6_ifdown initiates
>> a walk of the fib evicting routes with the 'lo' device which means all
>> host routes are removed. That process moves the dst which is attached to
>> an inet6_ifaddr to the gc list and marks it as dead.
>>
>> The recent change to keep global IPv6 addresses added a new function
>> fixup_permanent_addr that is called on admin up. That function restarts
>> dad for an inet6_ifaddr and when it completes the host route attached
>> to it is inserted into the fib. Since the route was marked dead and
>> moved to the gc list, we get the reported out-of-bounds accesses. If
>> the device with the address is taken down or the address is removed, the
>> WARN_ON in fib6_del is triggered.
>>
>> All of those faults are fixed by regenerating the host route of the
>> existing one has been moved to the gc list, something that can be
>> determined by checking if the rt6i_ref counter is 0.
>>
>> The update of the route on the ifp is done using cmpxchg as suggested
>> by Li RongQing in a patch a year ago.
>>
>> Fixes: f1705ec197e7 ("net: ipv6: Make address flushing on ifdown optional")
>> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>> Reported-by: Andrey Konovalov <andreyknvl@...gle.com>
>> Signed-off-by: David Ahern <dsa@...ulusnetworks.com>
>> ---
>>  net/ipv6/addrconf.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
>> index 08f9e8ea7a81..93555528a45b 100644
>> --- a/net/ipv6/addrconf.c
>> +++ b/net/ipv6/addrconf.c
>> @@ -3303,14 +3303,16 @@ static void addrconf_gre_config(struct net_device *dev)
>>  static int fixup_permanent_addr(struct inet6_dev *idev,
>>                               struct inet6_ifaddr *ifp)
>>  {
>> -     if (!ifp->rt) {
>> -             struct rt6_info *rt;
>> +     if (!ifp->rt || !atomic_read(&ifp->rt->rt6i_ref)) {
>> +             struct rt6_info *rt, *prev;
>>
>>               rt = addrconf_dst_alloc(idev, &ifp->addr, false);
>>               if (unlikely(IS_ERR(rt)))
>>                       return PTR_ERR(rt);
>>
>> -             ifp->rt = rt;
>> +             prev = cmpxchg(&ifp->rt, ifp->rt, rt);
> One small question.  Why cmpxchg is needed instead
> of a ip6_rt_put() and then assign?
> Is it fixing another bug?

cmpxchg here looks fishy.
If there are no concurrent modifications, then it is not needed.
If there are and cmpxchg fails, then we will put the installed rt and
leak the new one.




>> +             if (prev)
>> +                     ip6_rt_put(prev);
>>       }
>>
>>       if (!(ifp->flags & IFA_F_NOPREFIXROUTE)) {
>> --
>> 2.1.4
>>