linux-kernel - Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind

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Date:   Tue, 6 Jun 2023 16:28:02 +0200
From:   Mirsad Todorovac <mirsad.todorovac@....unizg.hr>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Shuah Khan <shuah@...nel.org>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind -
 ns-B IPv6 LLA" test

On 6/6/23 16:11, Guillaume Nault wrote:
> On Tue, Jun 06, 2023 at 03:57:35PM +0200, Mirsad Todorovac wrote:
>> diff --git a/net/ipv6/ping.c b/net/ipv6/ping.c
>> index c4835dbdfcff..c1d81c49b775 100644
>> --- a/net/ipv6/ping.c
>> +++ b/net/ipv6/ping.c
>> @@ -73,6 +73,10 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>          struct rt6_info *rt;
>>          struct pingfakehdr pfh;
>>          struct ipcm6_cookie ipc6;
>> +       struct net *net = sock_net(sk);
>> +       struct net_device *dev = NULL;
>> +       struct net_device *mdev = NULL;
>> +       struct net_device *bdev = NULL;
>>
>>          err = ping_common_sendmsg(AF_INET6, msg, len, &user_icmph,
>>                                    sizeof(user_icmph));
>> @@ -111,10 +115,26 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>          else if (!oif)
>>                  oif = np->ucast_oif;
>>
>> +       if (oif) {
>> +               rcu_read_lock();
>> +               dev = dev_get_by_index_rcu(net, oif);
>> +               rcu_read_unlock();
> 
> You can't assume '*dev' is still valid after rcu_read_unlock() unless
> you hold a reference on it.
> 
>> +               rtnl_lock();
>> +               mdev = netdev_master_upper_dev_get(dev);
>> +               rtnl_unlock();
> 
> Because of that, 'dev' might have already disappeared at the time
> netdev_master_upper_dev_get() is called. So it may dereference an
> invalid pointer here.

Good point, thanks. I didn't expect those to change.

This can be fixed, provided that RCU and RTNL locks can be nested:

         rcu_read_lock();
         if (oif) {
                 dev = dev_get_by_index_rcu(net, oif);
                 rtnl_lock();
                 mdev = netdev_master_upper_dev_get(dev);
                 rtnl_unlock();
         }

         if (sk->sk_bound_dev_if) {
                 bdev = dev_get_by_index_rcu(net, sk->sk_bound_dev_if);
         }

         addr_type = ipv6_addr_type(daddr);
         if ((__ipv6_addr_needs_scope_id(addr_type) && !oif) ||
             (addr_type & IPV6_ADDR_MAPPED) ||
             (oif && sk->sk_bound_dev_if && oif != sk->sk_bound_dev_if &&
                     !(mdev && sk->sk_bound_dev_if && bdev && mdev == bdev))) {
                 rcu_read_unlock();
                 return -EINVAL;
	}
         rcu_read_unlock();

But again this is still probably not race-free (bdev might also disappear before
the mdev == bdev test), even if it passed fcnal-test.sh, there is much duplication
of code, so your one-line solution is obviously by far better. :-)

Much obliged.

Best regards,
Mirsad

>> +       }
>> +
>> +       if (sk->sk_bound_dev_if) {
>> +               rcu_read_lock();
>> +               bdev = dev_get_by_index_rcu(net, sk->sk_bound_dev_if);
>> +               rcu_read_unlock();
>> +       }
>> +
>>          addr_type = ipv6_addr_type(daddr);
>>          if ((__ipv6_addr_needs_scope_id(addr_type) && !oif) ||
>>              (addr_type & IPV6_ADDR_MAPPED) ||
>> -           (oif && sk->sk_bound_dev_if && oif != sk->sk_bound_dev_if))
>> +           (oif && sk->sk_bound_dev_if && oif != sk->sk_bound_dev_if &&
>> +                   !(mdev && sk->sk_bound_dev_if && bdev && mdev == bdev)))
>>                  return -EINVAL;
>>
>>          ipcm6_init_sk(&ipc6, np);
>>
>> However, this works by the test (888 passed) but your two liner is obviously
>> better :-)
> 
> :)

-- 
Mirsad Goran Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu

System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia