linux-kernel - Re: [PATCH] net: 9p: Fix possible null-pointer dereference in p9_cm_event

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Date:   Tue, 10 Aug 2021 23:20:24 +0900
From:   asmadeus@...ewreck.org
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Tuo Li <islituo@...il.com>, ericvh@...il.com, lucho@...kov.net,
        davem@...emloft.net, kuba@...nel.org,
        v9fs-developer@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, baijiaju1990@...il.com,
        TOTE Robot <oslab@...nghua.edu.cn>
Subject: Re: [PATCH] net: 9p: Fix possible null-pointer dereference in
 p9_cm_event_handler()

Leon Romanovsky wrote on Tue, Aug 10, 2021 at 04:55:42PM +0300:
> On Tue, Aug 10, 2021 at 06:20:07AM -0700, Tuo Li wrote:
> > The variable rdma is checked when event->event is equal to 
> > RDMA_CM_EVENT_DISCONNECTED:
> >   if (rdma)
> > 
> > This indicates that it can be NULL. If so, a null-pointer dereference will 
> > occur when calling complete():
> >   complete(&rdma->cm_done);
> > 
> > To fix this possible null-pointer dereference, calling complete() only 
> > when rdma is not NULL.
> 
> You need to explain how is it possible and blindly set if () checks.
> I would say first "if (rdma)" is not needed, but don't know for sure.

Sounds like static analysis because there's a if (rdma) check in
RDMA_CM_EVENT_DISCONNECTED above, so if that needed check then it will
bug right afterwards

I'd tend to agree I don't think it's possible client->trans is null
there (it's filled right after rdma_create_id which defines the handler,
there might be a window where the callback is called before? But as I
understand it shouldn't be called until we resolve address and connect
then later disconnect)

So, I agree with Leon - unless you have a backtrace of a real bug
let's remove the other 'if' if you want to cleanup something for your
robot.

-- 
Dominique