linux-kernel - Re: mlxsw: spectrum: PTP: Support timestamping on Spectrum-1

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Message-ID: <e0f886ed-34cb-7b4b-a72c-726ca429bedd@canonical.com>
Date:   Tue, 2 Jul 2019 16:52:39 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Petr Machata <petrm@...lanox.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

On 02/07/2019 16:51, Petr Machata wrote:
> 
> 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.

Ah, that makes sense.

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