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

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Message-ID: <491430dd-71ad-4472-b3e1-0531da6d4ecc@redhat.com>
Date: Tue, 18 Mar 2025 11:29:15 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Qasim Ijaz <qasdev00@...il.com>, andrew+netdev@...n.ch,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org
Cc: 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() and
 cleanup error handling

On 3/11/25 5:11 PM, 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.
> 
> Furthermore the get_mac_address() function has a similar problem where
> it does not directly check the return value of each control_read(),
> instead it sums up the return values and checks them all at the end
> which means if any call to control_read() fails the function just 
> continues on.
> 
> Handle this by validating the return value of each call and fail fast
> and early instead of continuing.
> 
> Lastly ch9200_bind() ignores the return values of multiple 
> control_write() calls.
> 
> Validate each control_write() call to ensure it succeeds before
> continuing with the next call.
> 
> 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>

Please split the patch in a small series, as suggested by Simon.

Please additionally include the target tree name ('net', in this case)
in the subj prefix.

Thanks,

Paolo