Re: [PATCH 1/4] net: fix uninitialised access in mii_nway_restart()

[Simon Horman]

On Wed, Mar 19, 2025 at 11:21:53AM +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.
> 
> Reported-by: syzbot <syzbot+3361c2d6f78a3e0892f9@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Closes: https://syzkaller.appspot.com/bug?extid=3361c2d6f78a3e0892f9
> Tested-by: syzbot <syzbot+3361c2d6f78a3e0892f9@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Fixes: 4a476bd6d1d9 ("usbnet: New driver for QinHeng CH9200 devices")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Qasim Ijaz <qasdev00@xxxxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>