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

[Gustavo A. R. Silva]

On 8/10/21 11:30, Sakari Ailus wrote:
> On Tue, Aug 10, 2021 at 11:26:14AM -0500, Gustavo A. R. Silva wrote:
>> 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@xxxxxxxxxxxxxxx
>>>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
>>>>>> ---
>>>>>>
>>>>>> 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;
> 
> There's already a check the space required for the array of binary_nr is
> there. But not the number itself.

Yep; and that is for the upper limit.

The whole fix would be this:

diff --git a/drivers/staging/media/ipu3/ipu3-css-fw.c b/drivers/staging/media/ipu3/ipu3-css-fw.c
index 45aff76198e2..8830f42f2b12 100644
--- a/drivers/staging/media/ipu3/ipu3-css-fw.c
+++ b/drivers/staging/media/ipu3/ipu3-css-fw.c
@@ -124,12 +124,11 @@ int imgu_css_fw_init(struct imgu_css *css)
        /* Check and display fw header info */

        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;
-       if (sizeof(struct imgu_fw_bi_file_h) +
-           css->fwp->file_header.binary_nr * sizeof(struct imgu_fw_info) >
-           css->fw->size)
+       if (struct_size((struct imgu_fw_header *)0, binary_header,
+                       css->fwp->file_header.binary_nr) > css->fw->size)
                goto bad_fw;

        dev_info(dev, "loaded firmware version %.64s, %u binaries, %zu bytes\n",
diff --git a/drivers/staging/media/ipu3/ipu3-css-fw.h b/drivers/staging/media/ipu3/ipu3-css-fw.h
index 3c078f15a295..c0bc57fd678a 100644
--- a/drivers/staging/media/ipu3/ipu3-css-fw.h
+++ b/drivers/staging/media/ipu3/ipu3-css-fw.h
@@ -171,7 +171,7 @@ struct imgu_fw_bi_file_h {

 struct imgu_fw_header {
        struct imgu_fw_bi_file_h file_header;
-       struct imgu_fw_info binary_header[1];   /* binary_nr items */
+       struct imgu_fw_info binary_header[];    /* binary_nr items */
 };

 /******************* Firmware functions *******************/


Notice that "css->fw->size < struct_size(css->fwp, binary_header, 1)" with binary_header declared as a flexible-array
member is equivalent to "css->fw->size < sizeof(struct imgu_fw_header)" with binary_header declared as a one-element
array.

--
Gustavo