Re: [PATCH] HID: usbhid: fix out-of-bounds bug

[Alan Stern]

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.

> -	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.

> -	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++)

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)				\
>