Re: [PATCH] HID: usbhid: fix out-of-bounds bug
From: Jaejoong Kim
Date: Tue Sep 26 2017 - 15:59:59 EST
Hi, Alan,
Thanks for the review.
2017-09-26 23:18 GMT+09:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> 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,
diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
index 089bad8..0b41d44 100644
--- a/drivers/hid/usbhid/hid-core.c
+++ b/drivers/hid/usbhid/hid-core.c
@@ -975,6 +975,7 @@ static int usbhid_parse(struct hid_device *hid)
unsigned int rsize = 0;
char *rdesc;
int ret, n;
+ int num_descriptors;
quirks = usbhid_lookup_quirk(le16_to_cpu(dev->descriptor.idVendor),
le16_to_cpu(dev->descriptor.idProduct));
@@ -997,10 +998,17 @@ static int usbhid_parse(struct hid_device *hid)
return -ENODEV;
}
+ if (hdesc->bLength < sizeof(struct hid_descriptor)) {
+ dbg_hid("hid descriptor is too short\n");
+ return -EINVAL;
+ }
+
hid->version = le16_to_cpu(hdesc->bcdHID);
hid->country = hdesc->bCountryCode;
- for (n = 0; n < hdesc->bNumDescriptors; n++)
+ num_descriptors = min_t(int, hdesc->bNumDescriptors,
+ (hdesc->bLength - 6) / 3);
+ for (n = 0; n < num_descriptors; n++)
if (hdesc->desc[n].bDescriptorType == HID_DT_REPORT)
rsize = le16_to_cpu(hdesc->desc[n].wDescriptorLength);
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) \
>>
>