On Thu, Aug 29, 2013 at 1:59 AM, Benjamin Tissoires
[off-list ref] 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.

More specific comments inlined:

On 28/08/13 22:31, Jiri Kosina wrote:

quoted

From: Kees Cook <redacted>

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 <redacted>
Cc: stable@kernel.org
---
 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.

quoted

+                 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.

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.

quoted

+                     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.

quoted

              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 :(

quoted

+             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[].

Thanks for the review!

-Kees

Cheers,
Benjamin

quoted

      }

      for (r = 0; r < report->maxfield; r++) {

-- 
Kees Cook
Chrome OS Security