Re: [PATCH] flow_dissector: prevent NULL pointer dereference in __skb_flow_dissect

[Denis Kirjanov]

On 3/21/24 13:57, Jiri Pirko wrote:
> Thu, Mar 21, 2024 at 10:36:53AM CET, abelova@xxxxxxxxxxxxx wrote:
>>
>>
>> 20/03/24 16:38, Jiri Pirko пишет:
>>> Wed, Mar 20, 2024 at 01:56:35PM CET, abelova@xxxxxxxxxxxxx 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
> 
> Okay. So would it be possible to attach a link to the actual report?
> 
>>
>>
>>>> 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@xxxxxxxxxxxxx>
>>> Date:   Sat Aug 23 12:13:41 2014 -0700
>>>
>>>      net: Allow raw buffers to be passed into the flow dissector.
>>>
>>
>> Got it.

Looks like it's a static checker, there is no actual bug report or kernel oops/crash 

>>
>>>
>>>> Signed-off-by: Anastasia Belova <abelova@xxxxxxxxxxxxx>
>>>> ---
>>>> 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
>>
>