linux-kernel - Re: [PATCH 1/5] net: ch9200: fix uninitialised access bug during mii_nway

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Message-ID: <748d2e6c-68c0-43d4-8464-12e4942c7633@lunn.ch>
Date: Tue, 15 Apr 2025 03:28:50 +0200
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, horms@...nel.org,
	linux-usb@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzbot+3361c2d6f78a3e0892f9@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/5] net: ch9200: fix uninitialised access bug during
 mii_nway_restart

On Sat, Apr 12, 2025 at 07:38:25PM +0100, 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. We return 0 on control_read() failure here 
> because returning a negative may trigger the "bmcr & BMCR_ANENABLE" 
> check within mii_nway_restart.
> 
> Reported-by: syzbot <syzbot+3361c2d6f78a3e0892f9@...kaller.appspotmail.com>
> Closes: https://syzkaller.appspot.com/bug?extid=3361c2d6f78a3e0892f9
> Tested-by: syzbot <syzbot+3361c2d6f78a3e0892f9@...kaller.appspotmail.com>
> Fixes: 4a476bd6d1d9 ("usbnet: New driver for QinHeng CH9200 devices")
> Cc: stable@...r.kernel.org
> Signed-off-by: Qasim Ijaz <qasdev00@...il.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew