Re: KMSAN: uninit-value in cdc_ncm_set_dgram_size

Oliver Neukum
2019-11-05T11:31:21Z

Am Montag, den 04.11.2019, 22:22 +0100 schrieb Bjørn Mork:
> This looks like a false positive to me. max_datagram_size is two bytes
> declared as
>
> __le16 max_datagram_size;
>
> and the code leading up to the access on drivers/net/usb/cdc_ncm.c:587
> is:
>
> /* read current mtu value from device */
> err = usbnet_read_cmd(dev, USB_CDC_GET_MAX_DATAGRAM_SIZE,
> USB_TYPE_CLASS | USB_DIR_IN | USB_RECIP_INTERFACE,
> 0, iface_no, &max_datagram_size, 2);

At this point err can be 1.

> if (err < 0) {
> dev_dbg(&dev->intf->dev, "GET_MAX_DATAGRAM_SIZE failed\n");
> goto out;
> }
>
> if (le16_to_cpu(max_datagram_size) == ctx->max_datagram_size)
>
>
>
> AFAICS, there is no way max_datagram_size can be uninitialized here.
> usbnet_read_cmd() either read 2 bytes into it or returned an error,

No. usbnet_read_cmd() will return the number of bytes transfered up
to the number requested or an error.

> causing the access to be skipped. Or am I missing something?

Yes. You can get half the MTU. We have a similar class of bugs
with MAC addresses.

Regards
Oliver