netdev - Re: [PATCH v2] flow_dissector: prevent NULL pointer dereference in __skb_flow

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Date: Thu, 21 Mar 2024 15:52:10 +0300
From: Denis Kirjanov <dkirjanov@...e.de>
To: Anastasia Belova <abelova@...ralinux.ru>,
 "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH v2] flow_dissector: prevent NULL pointer dereference in
 __skb_flow_dissect

On 3/21/24 15:34, Anastasia Belova 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.
> 
> Fixes: 690e36e726d0 ("net: Allow raw buffers to be passed into the flow dissector.")
> Signed-off-by: Anastasia Belova <abelova@...ralinux.ru>

As request in the previous email please show the actual data flow that leads to a null pointer
dereference.
Also please read function description:
...
 * @skb: sk_buff to extract the flow from, can be NULL if the rest are specified
...

> ---
>  net/core/flow_dissector.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 272f09251343..68a8228ffae3 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -1139,6 +1139,8 @@ bool __skb_flow_dissect(const struct net *net,
>  
>  	if (dissector_uses_key(flow_dissector,
>  			       FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
> +		if (!skb)
> +			goto out_bad;
>  		struct ethhdr *eth = eth_hdr(skb);
>  		struct flow_dissector_key_eth_addrs *key_eth_addrs;
>