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

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Date:	Sat, 26 Jul 2014 11:44:57 +0100
From:	Malcolm Priestley <tvboxspy@...il.com>
To:	Guillaume CLÉMENT <gclement@...bob.org>
CC:	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 26/07/14 10:18, Guillaume CLÉMENT wrote:
> 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.
>

I got it wrong when the iw_handler is not present a standard ioctl is 
called extra is in userspace.

Sorry for the noise.
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