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

[Jiri Pirko]

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.


>
>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.



>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?


pw-bot: cr

>-- 
>2.30.2
>