linux-kernel - Re: [PATCH] staging: vt6655: fix direct dereferencing of user pointer

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Message-ID: <20140726091843.GA5956@pleinair.baobob.org>
Date:	Sat, 26 Jul 2014 11:18:45 +0200
From:	Guillaume CLÉMENT <gclement@...bob.org>
To:	Malcolm Priestley <tvboxspy@...il.com>
Cc:	Malcolm Priestley <tvboxspy@...il.com>,
	Forest Bond <forest@...ttletooquiet.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] staging: vt6655: fix direct dereferencing of user pointer

On Sat, Jul 26, 2014 at 10:24:30AM +0200, Guillaume CLÉMENT wrote:
> Hi Malcolm,
>
> On Sat, Jul 26, 2014 at 12:09:49AM +0100, Malcolm Priestley wrote:
> > Hi Guillaume
> >
> > On 25/07/14 13:47, Guillaume Clement wrote:
> > > Sparse reported that the data from tagSCmdRequest is given by
> > > userspace, so it should be tagged as such.
> > extra is not in user space
> >
>
> All right.
>
> This is still confusing to me because, taking the SIOCSIWGENIE ioctl as
> an example, in device_main.c, we have this code:
>
> rc = iwctl_siwgenie(dev, NULL, &(wrq->u.data), wrq->u.data.pointer);
>
> Here the extra parameter is the last one, wrq->u.data.pointer.
>
> I was led to believe that wrq->u.data.pointer is in userspace (this was
> reported by sparse actually) because the pointer field in data is
> actually defined as __user.
>
>
By the way, the original code (before my patch) reads:

	if ((wrq->length < 2) || (extra[1]+2 != wrq->length)) {
		ret = -EINVAL;
		goto out;
	}
	if (wrq->length > MAX_WPA_IE_LEN) {
		ret = -ENOMEM;
		goto out;
	}
	memset(pMgmt->abyWPAIE, 0, MAX_WPA_IE_LEN);
	if (copy_from_user(pMgmt->abyWPAIE, extra, wrq->length)) {
		ret = -EFAULT;
		goto out;
	}

Note extra[1] and later copy_from_user(x, extra, y).

If extra is not in userspace, we should not call copy_from_user, and if
it is, we should not dereference it. There is definitely something fishy
here.
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