linux-kernel - Re: [PATCH 4.19 094/133] USB: serial: iuu

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Message-ID: <20200722105247.GA19938@duo.ucw.cz>
Date:   Wed, 22 Jul 2020 12:52:47 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 4.19 094/133] USB: serial: iuu_phoenix: fix memory
 corruption

Hi!

> > > commit e7b931bee739e8a77ae216e613d3b99342b6dec0 upstream.
> > > 
> > > The driver would happily overwrite its write buffer with user data in
> > > 256 byte increments due to a removed buffer-space sanity check.
> > 
> > > +++ b/drivers/usb/serial/iuu_phoenix.c
> > > @@ -697,14 +697,16 @@ static int iuu_uart_write(struct tty_str
> > >  	struct iuu_private *priv = usb_get_serial_port_data(port);
> > >  	unsigned long flags;
> > >  
> > > -	if (count > 256)
> > > -		return -ENOMEM;
> > > -
> > >  	spin_lock_irqsave(&priv->lock, flags);
> > >  
> > > +	count = min(count, 256 - priv->writelen);
> > > +	if (count == 0)
> > > +		goto out;
> > > +
> > >  	/* fill the buffer */
> > >  	memcpy(priv->writebuf + priv->writelen, buf, count);
> > >  	priv->writelen += count;
> > > +out:
> > >  	spin_unlock_irqrestore(&priv->lock, flags);
> > >  
> > >  	return count;
> > 
> > Ok, so... goto and label is unneccessary, memcpy will do the right
> > thing with count == 0.
> 
> That's generally too subtle. Better to clearly mark the error/exception
> path.

We usually avoid subtle code by introducing comments, not by
introducing extra (and confusing) code that can not be optimized out.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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