Broadcom: Heap overflow in "wl_iw_get_essid" when handling WLC_GET_SSID ioctl results(CVE-2017-0570)

Broadcom produces Wi-Fi HardMAC SoCs which are used to handle the PHY and MAC layer processing. These chips are present in both mobile devices and Wi-Fi routers, and are capable of handling many Wi-Fi related events without delegating to the host OS. On Android devices, the “bcmdhd” driver is used in order to communicate with the Wi-Fi SoC (also referred to as “dongle”).

Along with the regular flow of frames transferred between the host and the dongle, the two communicate with one another via a set of “ioctls” which can be issued to read or write dongle configuration from the host. This information is exchanged using the SDIO “control” channel (SDPCM_CONTROL_CHANNEL) rather than the regular “data” and “glom” channels (which are used to transfer frames).

“bcmdhd” registers a set of handlers to support IW ioctls (which may be invoked on a socket descriptors where “iwreq.ifrname” points to a device entry exposed by the “bcmdhd” driver). These ioctls are handled by the “wl_iw_ioctl” function. The function first allocates an “extras” buffer of an appropriate size, according to the ioctl issued. Then, the function looks up the handler function matching the issued ioctl and invokes it, passing the allocated “extras” buffer as an argument.

When the “SIOCGIWESSID” ioctl is invoked (in order to query the current ESSID), the function allocates an extras buffer of size 33, and then invokes the internal handler function, “wl_iw_get_essid”. Here is a short snippet from the handler:

1.  wl_iw_get_essid(struct net_device *dev, struct iw_request_info *info,
2.                  struct iw_point *dwrq, char *extra)
3.  {
4.      wlc_ssid_t ssid;
5.      int error;
6.      if ((error = dev_wlc_ioctl(dev, WLC_GET_SSID, &ssid, sizeof(ssid)))) {
7.          WL_ERROR(("Error getting the SSID\n"));
8.          return error;
9.      }
10.     ssid.SSID_len = dtoh32(ssid.SSID_len);
11.     memcpy(extra, ssid.SSID, ssid.SSID_len);
12.     ...
13. }

Where “extra” is the extras buffer of size 33, allocated by “wl_iw_ioctl”.

As can be seen above, the handler trusts the value of “ssid.SSID_len”. An attacker controlling the dongle can re-write the ioctl handling function (since it is entirely RAM-resident), in order to control the results of the ioctl above. This would allow the attacker to return an arbitrarily large value for “ssid.SSID_len”, causing the memcpy operation (line 11) to overflow the “extra” buffer.

I’ve been able to statically verify this issue on the “bcmdhd-3.10” driver, and in the corresponding “bcmdhd” driver on the Nexus 5 (hammerhead) and Nexus 6P’s (angler) kernels.