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

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Message-ID: <cf0d2929-d854-48ce-97eb-69747f0833f2@lunn.ch>
Date: Tue, 18 Feb 2025 02:10:08 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Qasim Ijaz <qasdev00@...il.com>
Cc: andrew+netdev@...n.ch, 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,
	syzbot <syzbot+3361c2d6f78a3e0892f9@...kaller.appspotmail.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] net: fix uninitialised access in mii_nway_restart()

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?

	Andrew