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

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Date:   Tue, 2 Jul 2019 15:51:17 +0000
From:   Petr Machata <petrm@...lanox.com>
To:     Colin Ian King <colin.king@...onical.com>
CC:     Jiri Pirko <jiri@...lanox.com>, Ido Schimmel <idosch@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: mlxsw: spectrum: PTP: Support timestamping on Spectrum-1 -
 potential null ptr dereference


Colin Ian King <colin.king@...onical.com> writes:

> 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@...lanox.com>
> 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.

Yes, that line is wrong. It's missing a pair of parens, it should be:

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

I.e. I need a port && I need the skb->dev to still refer to that port
(or else be NULL). If that doesn't hold, bail out.

Thanks for the report, I'll spin a fix!