Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through phy_mii_ioctl()

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Tobias Waldekranz <tobias@waldekranz.com>
Cc: Andrew Lunn <andrew@lunn.ch>,
	Grygorii Strashko <grygorii.strashko@ti.com>,
	"David S. Miller" <davem@davemloft.net>,
	netdev@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	linux-kernel@vger.kernel.org,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Sean Anderson <sean.anderson@seco.com>
Subject: Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through phy_mii_ioctl()
Date: Thu, 4 Nov 2021 12:40:54 +0000	[thread overview]
Message-ID: <YYPU1gOvUPa00JWg@shell.armlinux.org.uk> (raw)
In-Reply-To: <YYPThd7aX+TBWslz@shell.armlinux.org.uk>

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.

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

-- 
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