linux-kernel - Re: [PATCH] net: fix uninitialised access in mii_nway

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Message-ID: <418ddcf6-e7c9-4a8e-ba1a-38a83cb2b5f8@lunn.ch>
Date: Wed, 26 Feb 2025 14:43:31 +0100
From: Andrew Lunn <andrew@...n.ch>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] net: fix uninitialised access in mii_nway_restart()

On Tue, Feb 18, 2025 at 12:19:57PM +0000, Qasim Ijaz wrote:
> On Tue, Feb 18, 2025 at 02:10:08AM +0100, Andrew Lunn wrote:
> > On Tue, Feb 18, 2025 at 12:24:43AM +0000, Qasim Ijaz wrote:
> > > In mii_nway_restart() during the line:
> > > 
> > > 	bmcr = mii->mdio_read(mii->dev, mii->phy_id, MII_BMCR);
> > > 
> > > The code attempts to call mii->mdio_read which is ch9200_mdio_read().
> > > 
> > > ch9200_mdio_read() utilises a local buffer, which is initialised 
> > > with control_read():
> > > 
> > > 	unsigned char buff[2];
> > > 	
> > > However buff is conditionally initialised inside control_read():
> > > 
> > > 	if (err == size) {
> > > 		memcpy(data, buf, size);
> > > 	}
> > > 
> > > If the condition of "err == size" is not met, then buff remains 
> > > uninitialised. Once this happens the uninitialised buff is accessed 
> > > and returned during ch9200_mdio_read():
> > > 
> > > 	return (buff[0] | buff[1] << 8);
> > > 	
> > > The problem stems from the fact that ch9200_mdio_read() ignores the
> > > return value of control_read(), leading to uinit-access of buff.
> > > 
> > > To fix this we should check the return value of control_read()
> > > and return early on error.
> > 
> > What about get_mac_address()?
> > 
> > If you find a bug, it is a good idea to look around and see if there
> > are any more instances of the same bug. I could be wrong, but it seems
> > like get_mac_address() suffers from the same problem?
> 
> Thank you for the feedback Andrew. I checked get_mac_address() before
> sending this patch and to me it looks like it does check the return value of
> control_read(). It accumulates the return value of each control_read() call into 
> rd_mac_len and then checks if it not equal to what is expected (ETH_ALEN which is 6),
> I believe each call should return 2.

It is unlikely a real device could trigger an issue, but a USB Rubber
Ducky might be able to. So the question is, are you interested in
protecting against malicious devices, or just making a static analyser
happy? Feel free to submit the patch as is.

	Andrew