netdev - Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through phy_mii

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Date:   Thu, 04 Nov 2021 14:13:08 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Grygorii Strashko <grygorii.strashko@...com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org,
        Vignesh Raghavendra <vigneshr@...com>,
        Sean Anderson <sean.anderson@...o.com>
Subject: Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through
 phy_mii_ioctl()

On Thu, Nov 04, 2021 at 12:40, "Russell King (Oracle)" <linux@...linux.org.uk> wrote:
> On Thu, Nov 04, 2021 at 12:35:17PM +0000, Russell King (Oracle) wrote:
>> On Thu, Nov 04, 2021 at 12:17:47PM +0100, Tobias Waldekranz wrote:
>> > Except that there is a way: https://github.com/wkz/mdio-tools
>> 
>> I'm guessing that this hasn't had much in the way of review, as it has
>> a nice exploitable bug - you really want "pc" to be unsigned in
>> mdio_nl_eval(), otherwise one can write a branch instruction that makes
>> "pc" negative.
>> 
>> Also it looks like one can easily exploit this to trigger any of your
>> BUG_ON()/BUG() statements, thereby crashing while holding the MDIO bus
>> lock causing a denial of service attack.
>> 
>> I also see nothing that protects against any user on a system being
>> able to use this interface, so the exploits above can be triggered by
>> any user. Moreover, this lack of protection means any user on the
>> system can use this interface to write to a PHY.
>> 
>> Given that some PHYs today contain firmware, this gives anyone access
>> to reprogram the PHY firmware, possibly introducing malicious firmware.
>> 
>> I hope no one is using this module in a production environment.
>
> It also leaks the reference count on the MDIO bus class device.
> mdio_find_bus(), rather class_find_device_by_name() takes a reference
> on the struct device that you never drop. See the documentation for
> class_find_device() for the statement about this:
>
>  * Note, you will need to drop the reference with put_device() after use.

Ahh interesting. Thanks!

> Of course, mdio_find_bus() documentation should _really_ have mentioned
> this fact too.

Yeah, that would have been helpful.