linux-kernel - Re: [PATCH 1/1] i2c: dev: check i2c_msg len before memdup_user() to prevent ZERO_SIZE

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Date:   Wed, 18 Apr 2018 09:07:04 +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>
Subject: Re: [PATCH 1/1] i2c: dev: check i2c_msg len before memdup_user() to
 prevent ZERO_SIZE_PTR deref

Hello,

On Wed, Apr 18, 2018 at 03:16:45AM +0300, Alexander Popov wrote:
> Currently i2cdev_ioctl_rdwr() doesn't check i2c_msg len against zero
> before calling memdup_user(). If this len is zero memdup_user() returns
> ZERO_SIZE_PTR, which is later considered as valid since
> IS_ERR(ZERO_SIZE_PTR) is false. That causes ZERO_SIZE_PTR deref oops.

You're saying that

	memdup_user(ptr, 0)

reads from *ptr? I'd say this is a bug in memdup_user, not its user.

If however the problem only happens later in

	if (msgs[i].flags & I2C_M_RECV_LEN) {
		if (!(msgs[i].flags & I2C_M_RD) || msgs[i].buf[0] < 1 || ...)

Your commit log is wrong (and I think the patch, too).

Best regards
Uwe

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