On Fri, Aug 30, 2013 at 8:27 AM, Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxx> wrote:
> On Thu, Aug 29, 2013 at 9:41 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Thu, Aug 29, 2013 at 1:59 AM, Benjamin Tissoires
>> <benjamin.tissoires@xxxxxxxxxx> wrote:
>>> Hi Kees,
>>>
>>> I would be curious to have the HID report descriptors (maybe off list)
>>> to understand how things can be that bad.
>>
>> Certainly! I'll send them your way. I did have to get pretty creative
>> to tickle these conditions.
>>
>>> On overall, I'd prefer all those checks to be in hid-core so that we
>>> have the guarantee that we don't have to open a new CVE each time a
>>> specific hid driver do not check for these ranges.
>>
>> I pondered doing this, but it seemed like something that needed wider
>> discussion, so I thought I'd start with just the dump of fixes. It
>> seems like the entire HID report interface should use access functions
>> to enforce range checking -- perhaps further enforced by making the
>> structure opaque to the drivers.
>
> The problem with access functions with range checking is when they are
> used at each input report. This will overload the kernel processing
> for static checks.
>
>>
>>> More specific comments inlined:
>>>
>>> On 28/08/13 22:31, Jiri Kosina wrote:
>>>> From: Kees Cook <keescook@xxxxxxxxxxxx>
>>>>
>>>> When working on report indexes, always validate that they are in bounds.
>>>> Without this, a HID device could report a malicious feature report that
>>>> could trick the driver into a heap overflow:
>>>>
>>>> [ 634.885003] usb 1-1: New USB device found, idVendor=0596, idProduct=0500
>>>> ...
>>>> [ 676.469629] BUG kmalloc-192 (Tainted: G W ): Redzone overwritten
>>>>
>>>> CVE-2013-2897
>>>>
>>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>>> Cc: stable@xxxxxxxxxx
>>>> ---
>>>> drivers/hid/hid-multitouch.c | 25 ++++++++++++++++++++-----
>>>> 1 file changed, 20 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
>>>> index cb0e361..2aa275e 100644
>>>> --- a/drivers/hid/hid-multitouch.c
>>>> +++ b/drivers/hid/hid-multitouch.c
>>>> @@ -330,9 +330,18 @@ static void mt_feature_mapping(struct hid_device *hdev,
>>>> break;
>>>> }
>>>> }
>>>> + /* Ignore if value index is out of bounds. */
>>>> + if (td->inputmode_index < 0 ||
>>>
>>> td->inputmode_index can not be less than 0
>>
>> Well, it certainly _shouldn't_ be less than zero. :) However, it is
>> defined as s8, and gets set from an int:
>>
>> for (i=0; i < field->maxusage; i++) {
>> if (field->usage[i].hid == usage->hid) {
>> td->inputmode_index = i;
>> break;
>> }
>> }
>>
>> Both "i" and "maxusage" are int, and I can generate a large maxusage
>> and usage array where the first matching hid equality happens when i
>> is >127, causing inputmode_index to wrap.
>
> ouch. Sorry. This piece of code (the current code) is junk. We should
> switch to usage->usage_index and change from __s8 to __s16 the
> declaration ASAP.
>
>>
>>>> + td->inputmode_index >= field->report_count) {
>>>
>>> if this is really required, we could just change the for loop above to
>>> go from 0 to field->report_count instead.
>>
>> That's certainly true, but since usage count and report count are not
>> directly associated, I don't know if there are devices that will freak
>> out with this restriction.
>
> Actually, I just again another long time understanding all the mess of
> having usage count and report count different in hid-core.
>
> I think it is a bug at some point (but it looks like I tend to be
> wrong on a lot of things recently :-P ), because when you look at the
> actual code of hid_input_field() in hid-core.c, we never go further
> than field->report_count when dealing with input report.
> So my guess is that we are declaring extra usages that are never used
> to parse incoming data.
>
>>
>>> However, I think we could just rely on usage->usage_index to get the
>>> actual index.
>>> I'll do more tests today and tell you later.
>>>
>>>> + dev_err(&hdev->dev, "HID_DG_INPUTMODE out of range\n");
>>>> + td->inputmode = -1;
>>>> + }
>>>>
>>>> break;
>>>> case HID_DG_CONTACTMAX:
>>>> + /* Ignore if value count is out of bounds. */
>>>> + if (field->report_count < 1)
>>>> + break;
>>>
>>> If you can trigger this, I would say that the fix should be in hid-core,
>>> not in every driver. A null report_count should not be allowed by the
>>> HID protocol.
>>
>> Again, I have no problem with that idea, but there seem to be lots of
>> general cases where this may not be possible (i.e. a HID with 0 OUTPUT
>> or FEATURE reports). I didn't see a sensible way to approach this in
>> core without declaring a "I require $foo many OUTPUT reports, $bar
>> many INPUT, and $baz many FEATURE" for each driver. I instead opted
>> for the report validation function instead.
>>
>
> I think we can just add this check in both hidinput_configure_usage()
> and report_features() (both in hid-input.c) so that we don't call the
> *_mapping() callbacks with non sense fields.
> This way, the specific hid drivers will know only the correct fields,
> and don't need to check this. So patches 9, 11 and 13 can be amended.
>
> It looks to me that you mismatched field->report_count and the actual
> report_count in your answer: field->report_count is the number of
> consecutive fields of field->report_size that are present for the
> parsing of the report. It has nothing to do with the total report
> count.
>
>>>> td->maxcontact_report_id = field->report->id;
>>>> td->maxcontacts = field->value[0];
>>>> if (!td->maxcontacts &&
>>>> @@ -743,15 +752,21 @@ static void mt_touch_report(struct hid_device *hid, struct hid_report *report)
>>>> unsigned count;
>>>> int r, n;
>>>>
>>>> + if (report->maxfield == 0)
>>>> + return;
>>>> +
>>>> /*
>>>> * Includes multi-packet support where subsequent
>>>> * packets are sent with zero contactcount.
>>>> */
>>>> - if (td->cc_index >= 0) {
>>>> - struct hid_field *field = report->field[td->cc_index];
>>>> - int value = field->value[td->cc_value_index];
>>>> - if (value)
>>>> - td->num_expected = value;
>>>> + if (td->cc_index >= 0 && td->cc_index < report->maxfield) {
>>>> + field = report->field[td->cc_index];
>>>
>>> looks like we previously overwrote the definition of field :(
>>>
>>>> + if (td->cc_value_index >= 0 &&
>>>> + td->cc_value_index < field->report_count) {
>>>> + int value = field->value[td->cc_value_index];
>>>> + if (value)
>>>> + td->num_expected = value;
>>>> + }
>>>
>>> I can not see why td->cc_index and td->cc_value_index could have bad
>>> values. They are initially created by hid-core, and are not provided by
>>> the device.
>>> Anyway, if you are able to produce bad values with them, I'd rather have
>>> those checks during the assignment of td->cc_index (in
>>> mt_touch_input_mapping()), instead of checking them for each report.
>>
>> Well, the problem comes again from there not being a hard link between
>> the usage array and the value array:
>>
>> td->cc_value_index = usage->usage_index;
>> ...
>> int value = field->value[td->cc_value_index];
>>
>> And all of this would be irrelevant if there was an access function
>> for field->value[].
>
> True, with the current implementation, td->cc_value_index can be
> greater than field->report_count. Still, moving this check in
> mt_touch_input_mapping() will allow us to make it only once.
>
>>
>> Thanks for the review!
>
> you are welcome :)
Okay, so, where does the whole patch series stand? It seems like the
checks are all worth adding; and my fixes are, I think, "minimal" so
having them go into the tree (so that they land in stable) seems like
a good idea. The work beyond that could be done on top of the existing
patches? There seems to be a lot of redesign ideas, but those probably
aren't so great for stable, and I'm probably not the best person to
write them, since everything I know about HID code I learned two weeks
ago. :)
Given the 12 flaws, what do you see as the best way forward?
-Kees
--
Kees Cook
Chrome OS Security
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