Re: [PATCH] net: dsa: felix: Add NULL check for outer_tagging_rule()

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Thu, Feb 13, 2025 at 12:07:54PM +0800, Wentao Liang wrote:
> In felix_update_tag_8021q_rx_rules(), the return value of
> ocelot_vcap_block_find_filter_by_id() is not checked, which could
> lead to a NULL pointer dereference if the filter is not found.
> 
> Add the necessary check and use `continue` to skip the current CPU
> port if the filter is not found, ensuring that all CPU ports are
> processed.
> 
> Fixes: f1288fd7293b ("net: dsa: felix: fix VLAN tag loss on CPU reception with ocelot-8021q")
> Cc: stable@vger.kernel.org # 6.11+
> Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
> ---
>  drivers/net/dsa/ocelot/felix.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/dsa/ocelot/felix.c b/drivers/net/dsa/ocelot/felix.c
> index 3aa9c997018a..10ad43108b88 100644
> --- a/drivers/net/dsa/ocelot/felix.c
> +++ b/drivers/net/dsa/ocelot/felix.c
> @@ -348,6 +348,8 @@ static int felix_update_tag_8021q_rx_rules(struct dsa_switch *ds, int port,
>  
>  		outer_tagging_rule = ocelot_vcap_block_find_filter_by_id(block_vcap_es0,
>  									 cookie, false);
> +		if (!outer_tagging_rule)
> +			continue;
>  
>  		felix_update_tag_8021q_rx_rule(outer_tagging_rule, vlan_filtering);
>  
> -- 
> 2.42.0.windows.2
>

Is this patch a result of static analysis or has a real problem been
noticed? Could you make a more detailed analysis for me of the code path
in which the filter cannot be found? because I'm not seeing it.

Hint: follow felix_change_tag_protocol(), and see which things are
guaranteed to have succeeded if felix->tag_proto == DSA_TAG_PROTO_OCELOT_8021Q.

felix_tag_8021q_vlan_add() is called from dsa_tag_8021q_register(), and
felix_tag_8021q_vlan_del() from dsa_tag_8021q_unregister().

felix_vlan_filtering() runs under rtnl_lock(), and so does felix_change_tag_protocol().
So felix_vlan_filtering() is always executed either entirely before, or
entirely after felix_change_tag_protocol(), never concurrently.