linux-kernel - Re: [v2 1/1] i2c: dev: prevent ZERO_SIZE_PTR deref in i2cdev_ioctl

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Date:   Fri, 20 Apr 2018 07:58:22 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Alexander Popov <alex.popov@...ux.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org, sil2review@...ts.osadl.org,
        Dmitry Vyukov <dvyukov@...gle.com>, syzkaller@...glegroups.com
Subject: Re: [v2 1/1] i2c: dev: prevent ZERO_SIZE_PTR deref in
 i2cdev_ioctl_rdwr()

Hello,

On Thu, Apr 19, 2018 at 08:01:46PM +0300, Alexander Popov wrote:
> On 19.04.2018 16:49, Uwe Kleine-König wrote:
> >> @@ -280,6 +280,7 @@ static noinline int i2cdev_ioctl_rdwr(struct i2c_client *client,
> >>  		 */
> >>  		if (msgs[i].flags & I2C_M_RECV_LEN) {
> >>  			if (!(msgs[i].flags & I2C_M_RD) ||
> >> +			    !msgs[i].len ||
> > 
> > I'd prefer
> > 
> > 			msgs[i].len > 0
> 
> Excuse me, it will be wrong. We stop if len is 0 to avoid the following
> ZERO_SIZE_PTR dereference.

right you are. I missed the negation.
 
> > here instead of
> > 
> > 			!msgs[i].len
> 
> I can change it to "msgs[i].len == 0". But is it really important?
> 
> I've carefully tested the current version with the original repro. It works correct.

I don't doubt it, and the code generated is maybe even the same. The
point I wanted to make is that

	!len

is harder to read for a human than

	len < 1

(or another suitable arithmetic expression). But feel free to disagree
and keep the code as is.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |