HID: usbhid: fix out-of-bounds bug

Hi, Alan,

Thanks for the review.

2017-09-26 23:18 GMT+09:00 Alan Stern <stern@rowland.harvard.edu>:
> On Tue, 26 Sep 2017, Jaejoong Kim wrote:
>
>> The starting address of the hid descriptor is obtained via
>> usb_get_extra_descriptor(). If the hid descriptor has the wrong size, it
>> is possible to access the wrong address. So, before accessing the hid
>> descriptor, we need to check the entire size through the bLength field.
>>
>> It also shows how many class descriptors it has through the bNumDescriptors
>> of the hid descriptor. Assuming that the connected hid descriptor has two
>> class descriptors(report and physical descriptors), the code below can
>> cause OOB because hdesc->desc is an array of size 1.
>>
>> for (n = 0; n < hdesc->bNumDescriptors; n++)
>>       if (hdesc->desc[n].bDescriptorType == HID_DT_REPORT)
>>               rsize = le16_to_cpu(hdesc->desc[n].wDescriptorLength);
>>
>> Since we know the starting address of the hid descriptor and the value of
>> the bNumDescriptors is variable, we directly access the buffer containing
>> the hid descriptor without usbing hdesc->desc to obtain the size of the
>> report descriptor.
>
> I do not like this patch at all.
>
>> diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
>> index 089bad8..7bad173 100644
>> --- a/drivers/hid/usbhid/hid-core.c
>> +++ b/drivers/hid/usbhid/hid-core.c
>> @@ -970,12 +970,19 @@ static int usbhid_parse(struct hid_device *hid)
>>       struct usb_interface *intf = to_usb_interface(hid->dev.parent);
>>       struct usb_host_interface *interface = intf->cur_altsetting;
>>       struct usb_device *dev = interface_to_usbdev (intf);
>> -     struct hid_descriptor *hdesc;
>> +     unsigned char *buffer = intf->altsetting->extra;
>> +     int buflen = intf->altsetting->extralen;
>> +     int length;
>>       u32 quirks = 0;
>>       unsigned int rsize = 0;
>>       char *rdesc;
>>       int ret, n;
>>
>> +     if (!buffer) {
>> +             dbg_hid("class descriptor not present\n");
>> +             return -ENODEV;
>> +     }
>> +
>>       quirks = usbhid_lookup_quirk(le16_to_cpu(dev->descriptor.idVendor),
>>                       le16_to_cpu(dev->descriptor.idProduct));
>>
>> @@ -990,19 +997,27 @@ static int usbhid_parse(struct hid_device *hid)
>>                               quirks |= HID_QUIRK_NOGET;
>>       }
>>
>> -     if (usb_get_extra_descriptor(interface, HID_DT_HID, &hdesc) &&
>> -         (!interface->desc.bNumEndpoints ||
>> -          usb_get_extra_descriptor(&interface->endpoint[0], HID_DT_HID, &hdesc))) {
>> -             dbg_hid("class descriptor not present\n");
>> -             return -ENODEV;
>> -     }
>> +     while (buflen > 2) {
>> +             length = buffer[0];
>> +             if (!length || length < HID_DESCRIPTOR_MIN_SIZE)
>> +                     goto next_desc;
>
> It's silly to duplicate the code that is already present in
> usb_get_extra_descriptor().  There's no reason to avoid using that
> function here.

To be honest, I was fooled. You are right. That is a duplicate code
with usb_get_extra_descriptor().

>
>> -     hid->version = le16_to_cpu(hdesc->bcdHID);
>> -     hid->country = hdesc->bCountryCode;
>> +             if (buffer[1] == HID_DT_HID) {
>> +                     hid->version = get_unaligned_le16(&buffer[2]);
>> +                     hid->country = buffer[4];
>
> It's also silly not to use the preformatted hid_descriptor structure and
> instead put error-prone byte offsets directly into the code.

Right.

>
>> -     for (n = 0; n < hdesc->bNumDescriptors; n++)
>> -             if (hdesc->desc[n].bDescriptorType == HID_DT_REPORT)
>> -                     rsize = le16_to_cpu(hdesc->desc[n].wDescriptorLength);
>> +                     for (n = 0; n < buffer[5]; n++) {
>> +                             /* we are just interested in report descriptor */
>> +                             if (buffer[6+3*n] != HID_DT_REPORT)
>> +                                     continue;
>> +                             rsize = get_unaligned_le16(&buffer[7+3*n]);
>
> Finally, this patch doesn't fix the actual problem!  You don't check
> here whether 8+3*n >= length.
>
> This whole thing could be fixed with a much smaller change to the
> original code.  Just do something like this:
>
>         num_descriptors = min_t(int, hdesc->bNumDescriptors,
>                         (hdesc->bLength - 6) / 3);
>         for (n = 0; n < num_descriptors; n++)

Right. Your change is better and clear.
With your change, I think we need check the size of hdesc before
accessing hdesc->desc[n] as Andrey Konovalov has already pointed out,

Jaejoong

>
> Alan Stern
>
>> +                     }
>> +             }
>> +
>> +next_desc:
>> +             buflen -= length;
>> +             buffer += length;
>> +     }
>>
>>       if (!rsize || rsize > HID_MAX_DESCRIPTOR_SIZE) {
>>               dbg_hid("weird size of report descriptor (%u)\n", rsize);
>> diff --git a/include/linux/hid.h b/include/linux/hid.h
>> index ab05a86..2d53c0f 100644
>> --- a/include/linux/hid.h
>> +++ b/include/linux/hid.h
>> @@ -638,6 +638,8 @@ struct hid_descriptor {
>>       struct hid_class_descriptor desc[1];
>>  } __attribute__ ((packed));
>>
>> +#define HID_DESCRIPTOR_MIN_SIZE      9
>> +
>>  #define HID_DEVICE(b, g, ven, prod)                                  \
>>       .bus = (b), .group = (g), .vendor = (ven), .product = (prod)
>>  #define HID_USB_DEVICE(ven, prod)                            \
>>
>
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HID: usbhid: fix out-of-bounds bug

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