netdev - Re: [PATCH net 0/8] net: fix uninit-values in networking stack

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Message-Id: <20180408.124959.590910633534141941.davem@davemloft.net>
Date:   Sun, 08 Apr 2018 12:49:59 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net 0/8] net: fix uninit-values in networking stack

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sun, 8 Apr 2018 09:38:13 -0700

> On 04/07/2018 07:40 PM, David Miller wrote:
>> From: Eric Dumazet <edumazet@...gle.com>
>> Date: Sat,  7 Apr 2018 13:42:35 -0700
>> 
>>> It seems syzbot got new features enabled, and fired some interesting
>>> reports. Oh well.
>> 
>> Series applied, however in patch #7 the condition syzbot detects
>> cannot happen.
>> 
>> In all code paths that lead to __mkroute_output() with res->type
>> uninitialized, __mkroute_output() will reassign the local variable
>> 'type' before reading it.
> 
> Well, we have :
> 
> u16 type = res->type;
> ...
> 
>        if (ipv4_is_lbcast(fl4->daddr))
>                 type = RTN_BROADCAST;
>         else if (ipv4_is_multicast(fl4->daddr))
>                 type = RTN_MULTICAST;
>         else if (ipv4_is_zeronet(fl4->daddr))
>                 return ERR_PTR(-EINVAL);
> 
> ...
> 
> if (type == RTN_BROADCAST) {  /* This is where KMSAN complained */
> 
> So it looks like type could have been random at this point.

Ok, then.  It seems that the requirement is:

	fl4->flowi4_oif is non-zero
	fl4->daddr is neither local multicast nor lbcast
	fl4->flowi4_proto is IPPROTO_IGMP

Then we can trigger such a sequence of events.