netdev - re: net: dsa: hellcreek: Add support for hardware timestamping

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Date:   Mon, 09 Nov 2020 14:59:13 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Colin Ian King <colin.king@...onical.com>,
        Kamil Alkhouri <kamil.alkhouri@...offenburg.de>,
        Richard Cochran <richardcochran@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        ivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors\@vger.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: re: net: dsa: hellcreek: Add support for hardware timestamping

Hi Colin,

On Mon Nov 09 2020, Colin Ian King wrote:
> Hi
>
> Static analysis on linux-next with Coverity has detected a potential
> null pointer dereference issue on the following commit:
>
> commit f0d4ba9eff75a79fccb7793f4d9f12303d458603
> Author: Kamil Alkhouri <kamil.alkhouri@...offenburg.de>
> Date:   Tue Nov 3 08:10:58 2020 +0100
>
>     net: dsa: hellcreek: Add support for hardware timestamping
>
> The analysis is as follows:
>
> 323                /* Get nanoseconds from ptp packet */
> 324                type = SKB_PTP_TYPE(skb);
>
>    4. returned_null: ptp_parse_header returns NULL (checked 10 out of 12
> times).
>    5. var_assigned: Assigning: hdr = NULL return value from
> ptp_parse_header.
>
> 325                hdr  = ptp_parse_header(skb, type);
>
>    Dereference null return value (NULL_RETURNS)
>    6. dereference: Dereferencing a pointer that might be NULL hdr when
> calling hellcreek_get_reserved_field.
>
> 326                ns   = hellcreek_get_reserved_field(hdr);
> 327                hellcreek_clear_reserved_field(hdr);
>
> This issue can only occur if the type & PTP_CLASS_PMASK is not one of
> PTP_CLASS_IPV4, PTP_CLASS_IPV6 or PTP_CLASS_L2.  I'm not sure if this is
> a possibility or not, but I'm assuming that it would be useful to
> perform the null check just in case, but I'm not sure how this affects
> the hw timestamping code in this function.

I don't see how the null pointer dereference could happen. That's the
Rx path you showed above.

The counter part code is:

hellcreek_port_rxtstamp:

	/* Make sure the message is a PTP message that needs to be timestamped
	 * and the interaction with the HW timestamping is enabled. If not, stop
	 * here
	 */
	hdr = hellcreek_should_tstamp(hellcreek, port, skb, type);
	if (!hdr)
		return false;

	SKB_PTP_TYPE(skb) = type;

Here the type is stored and hellcreek_should_tstamp() also calls
ptp_parse_header() internally. Only when ptp_parse_header() didn't
return NULL the first time the timestamping continues. It should be
safe.

Also the error handling would be interesting at that point. What should
happen if the header is null then? Returning an invalid timestamp?
Ignore it?

Hm. I think we have to make sure that it is a valid ptp packet before
reaching this code and that's what we've implemented. So, I guess it's
OK.

Thanks,
Kurt

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