CWE-1317: Improper Access Control in Fabric Bridge
Weakness ID: 1317
Vulnerability Mapping:ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction:
BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The product uses a fabric bridge for transactions between two Intellectual Property (IP) blocks, but the bridge does not properly perform the expected privilege, identity, or other access control checks between those IP blocks.
Extended Description
In hardware designs, different IP blocks are connected through interconnect-bus fabrics (e.g. AHB and OCP). Within a System on Chip (SoC), the IP block subsystems could be using different bus protocols. In such a case, the IP blocks are then linked to the central bus (and to other IP blocks) through a fabric bridge. Bridges are used as bus-interconnect-routing modules that link different protocols or separate, different segments of the overall SoC interconnect.
For overall system security, it is important that the access-control privileges associated with any fabric transaction are consistently maintained and applied, even when they are routed or translated by a fabric bridge. A bridge that is connected to a fabric without security features forwards transactions to the slave without checking the privilege level of the master and results in a weakness in SoC access-control security. The same weakness occurs if a bridge does not check the hardware identity of the transaction received from the slave interface of the bridge.
Common Consequences
This table specifies different individual consequences
associated with the weakness. The Scope identifies the application security area that is
violated, while the Impact describes the negative technical impact that arises if an
adversary succeeds in exploiting this weakness. The Likelihood provides information about
how likely the specific consequence is expected to be seen relative to the other
consequences in the list. For example, there may be high likelihood that a weakness will be
exploited to achieve a certain impact, but a low likelihood that it will be exploited to
achieve a different impact.
Scope: Confidentiality, Integrity, Access Control, AvailabilityLikelihood: Medium
Potential Mitigations
Phase(s)
Mitigation
Architecture and Design
Ensure that the design includes provisions for access-control checks in the bridge for both upstream and downstream transactions.
Implementation
Implement access-control checks in the bridge for both upstream and downstream transactions.
Relationships
This table shows the weaknesses and high level categories that are related to this
weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to
similar items that may exist at higher and lower levels of abstraction. In addition,
relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user
may want to explore.
Relevant to the view "Research Concepts" (View-1000)
Nature
Type
ID
Name
ChildOf
Pillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.
Peripherals, On-chip Fabric, and Interface/IO Problems
Modes
Of Introduction
The different Modes of Introduction provide information
about how and when this
weakness may be introduced. The Phase identifies a point in the life cycle at which
introduction
may occur, while the Note provides a typical scenario related to introduction during the
given
phase.
Phase
Note
Architecture and Design
Implementation
Applicable Platforms
This listing shows possible areas for which the given
weakness could appear. These
may be for specific named Languages, Operating Systems, Architectures, Paradigms,
Technologies,
or a class of such platforms. The platform is listed along with how frequently the given
weakness appears for that instance.
Languages
Class: Not Language-Specific
(Undetermined Prevalence)
Operating Systems
Class: Not OS-Specific
(Undetermined Prevalence)
Architectures
Class: Not Architecture-Specific
(Undetermined Prevalence)
Technologies
Processor Hardware
(Undetermined Prevalence)
Class: Not Technology-Specific
(Undetermined Prevalence)
Demonstrative Examples
Example 1
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 2
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.
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.
(good code)
Example Language: Verilog
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
Selected Observed
Examples
Note: this is a curated list of examples for users to understand the variety of ways in which this
weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.
Baseboard Management Controller (BMC) device implements Advanced High-performance Bus (AHB) bridges that do not require authentication for arbitrary read and write access to the BMC's physical address space from the host, and possibly the network [REF-1138].
Detection
Methods
Method
Details
Simulation / Emulation
RTL simulation to ensure that bridge-access controls are implemented properly.
Effectiveness: High
Formal Verification
Formal verification of bridge RTL to ensure that access control cannot be bypassed.
Effectiveness: High
Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that
reference this weakness as a member. This information is often useful in understanding where a
weakness fits within the context of external information sources.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
(this CWE ID may be used to map to real-world vulnerabilities)
Reason
Acceptable-Use
Rationale
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
Description
This table specifies different individual consequences
associated with the weakness. The Scope identifies the application security area that is
violated, while the Impact describes the negative technical impact that arises if an
adversary succeeds in exploiting this weakness. The Likelihood provides information about
how likely the specific consequence is expected to be seen relative to the other
consequences in the list. For example, there may be high likelihood that a weakness will be
exploited to achieve a certain impact, but a low likelihood that it will be exploited to
achieve a different impact.
