linux-hardening - Re: [PATCH v2 1/2] media: staging/intel-ipu3: css: Fix wrong size comparison

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Message-ID: <a5e19508-4d7a-ba63-7ac0-ed2e56bc3bc1@embeddedor.com>
Date:   Tue, 10 Aug 2021 11:26:14 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-kernel@...r.kernel.org, Yong Zhi <yong.zhi@...el.com>,
        Bingbu Cao <bingbu.cao@...el.com>,
        Tianshu Qiu <tian.shu.qiu@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-hardening@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2 1/2] media: staging/intel-ipu3: css: Fix wrong size
 comparison

Hi Sakari,

Please, see my comments below...

On 8/10/21 10:18, Sakari Ailus wrote:
> Hi Gustavo,
> 
> Apologies for the delay.
> 
> On Mon, Aug 02, 2021 at 08:46:20AM -0500, Gustavo A. R. Silva wrote:
>> Hi Sakari,
>>
>> On 8/2/21 01:05, Sakari Ailus wrote:
>>> Hi Gustavo,
>>>
>>> I missed you already had sent v2...
>>>
>>> On Fri, Jul 30, 2021 at 07:08:13AM -0500, Gustavo A. R. Silva wrote:
>>>> There is a wrong comparison of the total size of the loaded firmware
>>>> css->fw->size with the size of a pointer to struct imgu_fw_header.
>>>>
>>>> Fix this by using the right operand 'struct imgu_fw_header' for
>>>> sizeof, instead of 'struct imgu_fw_header *' and turn binary_header
>>>> into a flexible-array member. Also, adjust the relational operator
>>>> to be '<=' instead of '<', as it seems that the intention of the
>>>> comparison is to determine if the loaded firmware contains any
>>>> 'struct imgu_fw_info' items in the binary_header[] array than merely
>>>> the file_header (struct imgu_fw_bi_file_h).
>>>>
>>>> The replacement of the one-element array with a flexible-array member
>>>> also help with the ongoing efforts to globally enable -Warray-bounds
>>>> and get us closer to being able to tighten the FORTIFY_SOURCE routines
>>>> on memcpy().
>>>>
>>>> Link: https://github.com/KSPP/linux/issues/79
>>>> Link: https://github.com/KSPP/linux/issues/109
>>>> Fixes: 09d290f0ba21 ("media: staging/intel-ipu3: css: Add support for firmware management")
>>>> Cc: stable@...r.kernel.org
>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>>> ---
>>>>
>>>> It'd be just great if someone that knows this code better can confirm
>>>> these changes are correct. In particular the adjustment of the
>>>> relational operator. Thanks!
>>>>
>>>> Changes in v2:
>>>>  - Use flexible array and adjust relational operator, accordingly.
>>>
>>> The operator was just correct. The check is just there to see the firmware
>>> is at least as large as the struct as which it is being accessed.
>>
>> I'm a bit confused, so based on your reply to v1 of this series, this patch
>> is now correct, right?
>>
>> The operator in v1 _was_ correct as long as the one-element array wasn't
>> transformed into a flexible array, right?
>>
>> Notice that generally speaking flexible-array members don't occupy space in the
>> containing structure:
>>
>> $ pahole -C imgu_fw_header drivers/staging/media/ipu3/ipu3-css-fw.o
>> struct imgu_fw_header {
>> 	struct imgu_fw_bi_file_h   file_header;          /*     0    72 */
>> 	/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
>> 	struct imgu_fw_info        binary_header[] __attribute__((__aligned__(8))); /*    72     0 */
>>
>> 	/* size: 72, cachelines: 2, members: 2 */
>> 	/* forced alignments: 1 */
>> 	/* last cacheline: 8 bytes */
>> } __attribute__((__aligned__(8)));
>>
>> $ pahole -C imgu_fw_header drivers/staging/media/ipu3/ipu3-css-fw.o
>> struct imgu_fw_header {
>> 	struct imgu_fw_bi_file_h   file_header;          /*     0    72 */
>> 	/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
>> 	struct imgu_fw_info        binary_header[1] __attribute__((__aligned__(8))); /*    72  1200 */
>>
>> 	/* size: 1272, cachelines: 20, members: 2 */
>> 	/* forced alignments: 1 */
>> 	/* last cacheline: 56 bytes */
>> } __attribute__((__aligned__(8)));
>>
>> So, now that the flexible array transformation is included in the same patch as the
>> bugfix, the operator is changed from '<' to '<='
> 
> '<' is correct since you only need as much data as the struct you're about
> to access is large, not a byte more than that. As Dan noted.
> 
> I think you could add a check for binary_nr is at least one.

If we need to check that binary_nr is at least one, then this would be the right
change:

        css->fwp = (struct imgu_fw_header *)css->fw->data;
-       if (css->fw->size < sizeof(struct imgu_fw_header *) ||
+       if (css->fw->size < struct_size(css->fwp, binary_header, 1) ||
            css->fwp->file_header.h_size != sizeof(struct imgu_fw_bi_file_h))
                goto bad_fw;


--
Gustavo