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

From: Jiri Pirko <[email-masked]>
Date: Thu, 21 Mar 2024 11:57:30 +0100

Thu, Mar 21, 2024 at 10:36:53AM CET, [email-masked] wrote:
>
>
>20/03/24 16:38, Jiri Pirko пишет:
>> Wed, Mar 20, 2024 at 01:56:35PM CET, [email-masked] 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 <[email-masked]>
>> 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 <[email-masked]>
>> > ---
>> > 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
>