Re: [PATCH v2] flow_dissector: prevent NULL pointer dereference in __skb_flow_dissect

[Eric Dumazet]

On Thu, Mar 21, 2024 at 1:35 PM Anastasia Belova <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.
>
> Fixes: 690e36e726d0 ("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, 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;
>


I think you ignored my prior feedback.

In which case can we go to this point with skb == NULL ?
How come nobody complained of crashes here ?

I think we need to know if adding code here is useful or not.

You have to understand that a patch like this might need days of work
from various teams in the world,
flooded by questionable CVE.