linux-kernel - Re: [PATCH net v2] tipc: call tipc_rcv() only if bearer is up in tipc_udp

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Date:   Fri, 1 Dec 2017 16:02:47 +0200
From:   Tommi Rantala <tommi.t.rantala@...ia.com>
To:     Ying Xue <ying.xue@...driver.com>,
        Jon Maloy <jon.maloy@...csson.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        tipc-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] tipc: call tipc_rcv() only if bearer is up in
 tipc_udp_recv()

On 01.12.2017 15:18, Ying Xue wrote:
> On 11/30/2017 08:32 PM, Tommi Rantala wrote:
>>> In my opinion, the real root cause of the issue is because we too early
>>> set a not-yet-initialized bearer instance to ub->bearer through
>>> rcu_assign_pointer(ub->bearer, b) in tipc_udp_enable(). Instead if we
>>> assign the bearer pointer at the end of tipc_udp_enable() where the
>>> bearer has been completed the initialization, the issue would be avoided.
>> Hi, sorry, I fail to see how that helps.
>>
>> bearer->tolerance is only initialized in tipc_enable_bearer() after
>> calling m->enable_media() ie. tipc_udp_enable().
>>
>> So even if we do "rcu_assign_pointer(ub->bearer, b)" later in
>> tipc_udp_enable(), bearer->tolerance will still be uninitialized, and
>> the crash can happen.
> 
> Sorry, I missed the point that b->tolerance is not uninitialized when we
> assign bearer pointer to ub->bearer later.
> 
> But in my view the issue happened is because we enable media too early.
> So it's better to change the code as belows:

Thanks, looks good to me!

Tested in 4.4 (which does not have b->up), and this fixes the oops also 
there.

-Tommi


> diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c
> index 47ec121..ec6f02a 100644
> --- a/net/tipc/bearer.c
> +++ b/net/tipc/bearer.c
> @@ -320,19 +320,18 @@ static int tipc_enable_bearer(struct net *net,
> const char *name,
> 
>          strcpy(b->name, name);
>          b->media = m;
> -       res = m->enable_media(net, b, attr);
> -       if (res) {
> -               pr_warn("Bearer <%s> rejected, enable failure (%d)\n",
> -                       name, -res);
> -               return -EINVAL;
> -       }
> -
>          b->identity = bearer_id;
>          b->tolerance = m->tolerance;
>          b->window = m->window;
>          b->domain = disc_domain;
>          b->net_plane = bearer_id + 'A';
>          b->priority = priority;
> +       res = m->enable_media(net, b, attr);
> +       if (res) {
> +               pr_warn("Bearer <%s> rejected, enable failure (%d)\n",
> +                       name, -res);
> +               return -EINVAL;
> +       }
>          test_and_set_bit_lock(0, &b->up);
>