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

[Jiri Pirko <jiri@resnulli.us>]

Wed, Mar 20, 2024 at 02:43:22PM CET, edumazet@google.com wrote:
>On Wed, Mar 20, 2024 at 2:38 PM Jiri Pirko <jiri@resnulli.us> wrote:
>>
>> Wed, Mar 20, 2024 at 01:56:35PM CET, abelova@astralinux.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.
>>
>>
>> >
>> >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@davemloft.net>
>> Date:   Sat Aug 23 12:13:41 2014 -0700
>>
>>     net: Allow raw buffers to be passed into the flow dissector.
>>
>>
>>
>> >Signed-off-by: Anastasia Belova <abelova@astralinux.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?
>>
>
>It would be nice knowing in which context we could have a NULL skb and
>FLOW_DISSECTOR_KEY_ETH_ADDRS
>at the same time.
>
>It seems this fix is based on some kind of static analysis, but no real bug.

Yeah, I agree. That's the main reason I asked for the link to the report.