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 21:18:51 +0800
From:   Ying Xue <ying.xue@...driver.com>
To:     Tommi Rantala <tommi.t.rantala@...ia.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 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:

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);