linux-kernel - Re: [PATCH v3 1/4] ozwpan: Use proper check to prevent heap overflow

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Message-ID: <CAH6sp9P2_dOF=Hh0Y+tMLNuUdcmJ+BkYDQh+LHgBdikVcmaRxg@mail.gmail.com>
Date:	Fri, 29 May 2015 14:36:36 +0200
From:	Frans Klaver <fransklaver@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	"Jason A. Donenfeld" <Jason@...c4.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	devel@...verdev.osuosl.org,
	Shigekatsu Tateno <shigekatsu.tateno@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 1/4] ozwpan: Use proper check to prevent heap overflow

Hi,

On Fri, May 29, 2015 at 2:00 PM, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> On Fri, May 29, 2015 at 01:06:58PM +0200, Jason A. Donenfeld wrote:
>> --- a/drivers/staging/ozwpan/ozusbsvc1.c
>> +++ b/drivers/staging/ozwpan/ozusbsvc1.c
>> @@ -390,10 +390,15 @@ void oz_usb_rx(struct oz_pd *pd, struct oz_elt *elt)
>>       case OZ_GET_DESC_RSP: {
>>                       struct oz_get_desc_rsp *body =
>>                               (struct oz_get_desc_rsp *)usb_hdr;
>> -                     int data_len = elt->length -
>> -                                     sizeof(struct oz_get_desc_rsp) + 1;
>> -                     u16 offs = le16_to_cpu(get_unaligned(&body->offset));
>> -                     u16 total_size =
>> +                     u16 offs, total_size;
>> +                     u8 data_len;
>> +
>> +                     if (elt->length < sizeof(struct oz_get_desc_rsp) - 1)
>> +                             break;
>> +                     data_len = elt->length -
>> +                                     (sizeof(struct oz_get_desc_rsp) - 1);
>
> Gar...  I'm really sorry.  I wanted to Ack these and be done but why did
> the + 1 change to a - 1?  And I had the same question about the other
> patch as well.

I would say that it is because part of the expression has been placed
inside parentheses:

    a - b + 1 == a - (b - 1)

Guess it makes the decision logic slightly more readable.

Frans
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