Improper Access Control in Fabric Bridge

Description

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This example is from CVE-2019-6260 [REF-1138]. The iLPC2AHB bridge connects a CPU (with multiple, privilege levels, such as user, super user, debug, etc.) over AHB interface to an LPC bus. Several peripherals are connected to the LPC bus. The bridge is expected to check the privilege level of the transactions initiated in the core before forwarding them to the peripherals on the LPC bus.

The bridge does not implement the checks and allows reads and writes from all privilege levels.

To address this, designers should implement hardware-based checks that are either hardcoded to block untrusted agents from accessing secure peripherals or implement firmware flows that configure the bridge to block untrusted agents from making arbitrary reads or writes.

Example Two

The example code below is taken from the AES and core local interrupt (CLINT) peripherals of the HACK@DAC'21 buggy OpenPiton SoC. The access to all the peripherals for a given privilege level of the processor is controlled by an access control module in the SoC. This ensures that malicious users with insufficient privileges do not get access to sensitive data, such as the AES keys used by the operating system to encrypt and decrypt information. The security of the entire system will be compromised if the access controls are incorrectly enforced. The access controls are enforced through the interconnect-bus fabrics, where access requests with insufficient access control permissions will be rejected.

...
module aes0_wrapper #(...)(...);
...

  input logic     acct_ctrl_i;

...

  axi_lite_interface #(...
  ) axi_lite_interface_i (
  ...

    .en_o       ( en_acct    ),


...
..);

  assign en = en_acct && acct_ctrl_i;

...
endmodule
...
module clint #(...)(...);
...

  axi_lite_interface #(...
  ) axi_lite_interface_i (
  ...

    .en_o       ( en         ),


...
);
...
endmodule

The previous code snippet [REF-1382] illustrates an instance of a vulnerable implementation of access control for the CLINT peripheral (see module clint). It also shows a correct implementation of access control for the AES peripheral (see module aes0_wrapper) [REF-1381]. An enable signal (en_o) from the fabric's AXI interface (present in both modules) is used to determine if an access request is made to the peripheral. In the case of the AES peripheral, this en_o signal is first received in a temporary signal en_acct. Then, the access request is enabled (by asserting the en signal) only if the request has sufficient access permissions (i.e., acct_ctrl_i signal should be enabled). However, in the case of the CLINT peripheral, the enable signal, en_o, from the AXI interface, is directly used to enable accesses. As a result, users with insufficient access permissions also get full access to the CLINT peripheral.

To fix this, enable access requests to CLINT [REF-1383] only if the user has sufficient access as indicated by the acct_ctrl_i signal in the boolean && with en_acct.

module clint #(...
) (
...

  input logic     acct_ctrl_i,

...
);

  logic     en  , en_acct  ;

...

  axi_lite_interface #(...
  ) axi_lite_interface_i (

...

  .en_o       (  en_acct  ),

...

  );
  assign en = en_acct && acct_ctrl_i;

...
endmodule

See Also