re: mlxsw: spectrum: PTP: Support timestamping on Spectrum-1 - potential null ptr dereference

[Colin Ian King]

Hi,

Static analysis with Coverity on today's linux-next has found a
potential null pointer dereference bug with the following commit:

commit d92e4e6e33c8b19635be70fb8935b627d2e4f8fe
Author: Petr Machata <petrm@xxxxxxxxxxxx>
Date:   Sun Jun 30 09:04:56 2019 +0300

    mlxsw: spectrum: PTP: Support timestamping on Spectrum-1


In function: mlxsw_sp1_ptp_packet_finish the offending code is as follows:

       /* Between capturing the packet and finishing it, there is a
window of
        * opportunity for the originating port to go away (e.g. due to a
        * split). Also make sure the SKB device reference is still valid.
        */
       mlxsw_sp_port = mlxsw_sp->ports[local_port];
       if (!mlxsw_sp_port && (!skb->dev || skb->dev ==
mlxsw_sp_port->dev)) {
               dev_kfree_skb_any(skb);
               return;
       }

If mlxsw_sp_port is null and skb->dev is not-null then the comparison
"skb->dev == mlxsw_sp_port->dev" ends up with a null pointer dereference.

I think the if statement should be:

if (mlxsw_sp_port && (!skb->dev || skb->dev == mlxsw_sp_port->dev))

..but I'm not 100% sure as I may be missing something a bit more subtle
here.

Colin