netdev - Re: [PATCH] flow_dissector: prevent NULL pointer dereference in __skb_flow

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Date: Thu, 21 Mar 2024 12:36:53 +0300
From: Anastasia Belova <abelova@...ralinux.ru>
To: Jiri Pirko <jiri@...nulli.us>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 lvc-project@...uxtesting.org
Subject: Re: [PATCH] flow_dissector: prevent NULL pointer dereference in
 __skb_flow_dissect



20/03/24 16:38, Jiri Pirko пишет:
> Wed, Mar 20, 2024 at 01:56:35PM CET, abelova@...ralinux.ru wrote:
>> skb is an optional parameter, so it may be NULL.
>> Add check defore dereference in eth_hdr.
>>
>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> Either drop this line which provides no value, or attach a link to the
> actual report.
>

It is an established practice for our project. You can find 700+ applied
patches with similar line:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?qt=grep&q=linuxtesting.org


>> Fixes: 67a900cc0436 ("flow_dissector: introduce support for Ethernet addresses")
> This looks incorrect. I believe that this is the offending commit:
> commit 690e36e726d00d2528bc569809048adf61550d80
> Author: David S. Miller <davem@...emloft.net>
> Date:   Sat Aug 23 12:13:41 2014 -0700
>
>      net: Allow raw buffers to be passed into the flow dissector.
>

Got it.

>
>> Signed-off-by: Anastasia Belova <abelova@...ralinux.ru>
>> ---
>> net/core/flow_dissector.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
>> index 272f09251343..05db3a8aa771 100644
>> --- a/net/core/flow_dissector.c
>> +++ b/net/core/flow_dissector.c
>> @@ -1137,7 +1137,7 @@ bool __skb_flow_dissect(const struct net *net,
>> 		rcu_read_unlock();
>> 	}
>>
>> -	if (dissector_uses_key(flow_dissector,
>> +	if (skb && dissector_uses_key(flow_dissector,
>> 			       FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
>> 		struct ethhdr *eth = eth_hdr(skb);
>> 		struct flow_dissector_key_eth_addrs *key_eth_addrs;
> Looks like FLOW_DISSECT_RET_OUT_BAD should be returned in case the
> FLOW_DISSECTOR_KEY_ETH_ADDRS are selected and there is no skb, no?

I agree, I'll send the second version.

Anastasia Belova