CWE - CWE-1262: Improper Access Control for Register Interface (4.16)

Weakness ID: 1262

Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - 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 memory-mapped I/O registers that act as an interface to hardware functionality from software, but there is improper access control to those registers.

Extended Description

Software commonly accesses peripherals in a System-on-Chip (SoC) or other device through a memory-mapped register interface. Malicious software could tamper with any security-critical hardware data that is accessible directly or indirectly through the register interface, which could lead to a loss of confidentiality and integrity.

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	Impact	Likelihood
Confidentiality Integrity	Technical Impact: Read Memory; Read Application Data; Modify Memory; Modify Application Data; Gain Privileges or Assume Identity; Bypass Protection Mechanism; Unexpected State; Alter Execution Logic Confidentiality of hardware assets may be violated if the protected information can be read out by software through the register interface. Registers storing security state, settings, other security-critical data may be corruptible by software without correctly implemented protections.

Potential Mitigations

Phase: Architecture and Design

Design proper policies for hardware register access from software.

Phase: Implementation

Ensure that access control policies for register access are implemented in accordance with the specified design.

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" (CWE-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.	284	Improper Access Control

Relevant to the view "Hardware Design" (CWE-1194)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1198	Privilege Separation and Access Control Issues

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	This weakness may be exploited if the register interface design does not adequately protect hardware assets from software.
Implementation	Mis-implementation of access control policies may inadvertently allow access to hardware assets through the register interface.

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

Class: Not Technology-Specific (Undetermined Prevalence)

Demonstrative Examples

Example 1

The register interface provides software access to hardware functionality. This functionality is an attack surface. This attack surface may be used to run untrusted code on the system through the register interface. As an example, cryptographic accelerators require a mechanism for software to select modes of operation and to provide plaintext or ciphertext data to be encrypted or decrypted as well as other functions. This functionality is commonly provided through registers.

(bad code)

Cryptographic key material stored in registers inside the cryptographic accelerator can be accessed by software.

(good code)

Key material stored in registers should never be accessible to software. Even if software can provide a key, all read-back paths to software should be disabled.

Example 2

The example code is taken from the Control/Status Register (CSR) module inside the processor core of the HACK@DAC'19 buggy CVA6 SoC [REF-1340]. In RISC-V ISA [REF-1341], the CSR file contains different sets of registers with different privilege levels, e.g., user mode (U), supervisor mode (S), hypervisor mode (H), machine mode (M), and debug mode (D), with different read-write policies, read-only (RO) and read-write (RW). For example, machine mode, which is the highest privilege mode in a RISC-V system, registers should not be accessible in user, supervisor, or hypervisor modes.

(bad code)

Example Language: Verilog

if (csr_we || csr_read) begin

if ((riscv::priv_lvl_t'(priv_lvl_o & csr_addr.csr_decode.priv_lvl) != csr_addr.csr_decode.priv_lvl) && !(csr_addr.address==riscv::CSR_MEPC)) begin

csr_exception_o.cause = riscv::ILLEGAL_INSTR;
csr_exception_o.valid = 1'b1;

end
// check access to debug mode only CSRs
if (csr_addr_i[11:4] == 8'h7b && !debug_mode_q) begin

csr_exception_o.cause = riscv::ILLEGAL_INSTR;
csr_exception_o.valid = 1'b1;

end

The vulnerable example code allows the machine exception program counter (MEPC) register to be accessed from a user mode program by excluding the MEPC from the access control check. MEPC as per the RISC-V specification can be only written or read by machine mode code. Thus, the attacker in the user mode can run code in machine mode privilege (privilege escalation).

To mitigate the issue, fix the privilege check so that it throws an Illegal Instruction Exception for user mode accesses to the MEPC register. [REF-1345]

(good code)

Example Language: Verilog

if (csr_we || csr_read) begin

if ((riscv::priv_lvl_t'(priv_lvl_o & csr_addr.csr_decode.priv_lvl) != csr_addr.csr_decode.priv_lvl)) begin

csr_exception_o.cause = riscv::ILLEGAL_INSTR;
csr_exception_o.valid = 1'b1;

end
// check access to debug mode only CSRs
if (csr_addr_i[11:4] == 8'h7b && !debug_mode_q) begin

csr_exception_o.cause = riscv::ILLEGAL_INSTR;
csr_exception_o.valid = 1'b1;

end

Observed Examples

Reference	Description
CVE-2014-2915	virtualization product does not restrict access to debug and other processor registers in the hardware, allowing a crash of the host or guest OS
CVE-2021-3011	virtual interrupt controller in a virtualization product allows crash of host by writing a certain invalid value to a register, which triggers a fatal error instead of returning an error code
CVE-2020-12446	Driver exposes access to Model Specific Register (MSR) registers, allowing admin privileges.
CVE-2015-2150	Virtualization product does not restrict access to PCI command registers, allowing host crash from the guest.

Weakness Ordinalities

Ordinality	Description
Primary	(where the weakness exists independent of other weaknesses)

Detection Methods

Manual Analysis

This is applicable in the Architecture phase before implementation started. Make sure access policy is specified for the entire memory map. Manual analysis may not ensure the implementation is correct.

Effectiveness: Moderate

Manual Analysis

Registers controlling hardware should have access control implemented. This access control may be checked manually for correct implementation. Items to check consist of how are trusted parties set, how are trusted parties verified, how are accesses verified, etc. Effectiveness of a manual analysis will vary depending upon how complicated the interface is constructed.

Effectiveness: Moderate

Simulation / Emulation

Functional simulation is applicable during the Implementation Phase. Testcases must be created and executed for memory mapped registers to verify adherence to the access control policy. This method can be effective, since functional verification needs to be performed on the design, and verification for this weakness will be included. There can be difficulty covering the entire memory space during the test.

Effectiveness: Moderate

Formal Verification

Formal verification is applicable during the Implementation phase. Assertions need to be created in order to capture illegal register access scenarios and prove that they cannot occur. Formal methods are exhaustive and can be very effective, but creating the cases for large designs may be complex and difficult.

Effectiveness: High

Automated Analysis

Information flow tracking can be applicable during the Implementation phase. Security sensitive data (assets) - for example, as stored in registers - is automatically tracked over time through the design to verify the data doesn't reach illegal destinations that violate the access policies for the memory map. This method can be very effective when used together with simulation and emulation, since detecting violations doesn't rely on specific scenarios or data values. This method does rely on simulation and emulation, so testcases must exist in order to use this method.

Effectiveness: High

Architecture or Design Review

Manual documentation review of the system memory map, register specification, and permissions associated with accessing security-relevant functionality exposed via memory-mapped registers.

Effectiveness: Moderate

Fuzzing

Perform penetration testing (either manual or semi-automated with fuzzing) to verify that access control mechanisms such as the memory protection units or on-chip bus firewall settings adequately protect critical hardware registers from software access.

Effectiveness: Moderate

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.	1396	Comprehensive Categorization: Access Control

Vulnerability Mapping Notes

Usage: ALLOWED

(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.

Related Attack Patterns

CAPEC-ID	Attack Pattern Name
CAPEC-680	Exploitation of Improperly Controlled Registers

References

[REF-1340] "Hackatdac19 csr_regfile.sv". 2019. <https://github.com/HACK-EVENT/hackatdac19/blob/619e9fb0ef32ee1e01ad76b8732a156572c65700/src/csr_regfile.sv#L854:L857>. URL validated: 2023-06-21.

[REF-1341] Andrew Waterman, Yunsup Lee, Rimas Avižienis, David Patterson and Krste Asanović. "The RISC-V Instruction Set Manual". Volume II: Privileged Architecture. 2016-11-04. <https://people.eecs.berkeley.edu/~krste/papers/riscv-privileged-v1.9.1.pdf>. URL validated: 2023-06-21.

[REF-1345] Florian Zaruba, Michael Schaffner and Andreas Traber. "csr_regfile.sv". 2019. <https://github.com/openhwgroup/cva6/blob/7951802a0147aedb21e8f2f6dc1e1e9c4ee857a2/src/csr_regfile.sv#L868:L871>. URL validated: 2023-06-21.

Content History

Submissions
Submission Date	Submitter	Organization
2020-05-08 (CWE 4.1, 2020-02-24)	Nicole Fern	Cycuity (originally submitted as Tortuga Logic)
2020-05-08 (CWE 4.1, 2020-02-24)
Contributions
Contribution Date	Contributor	Organization
2021-10-11	Anders Nordstrom, Alric Althoff	Cycuity (originally submitted as Tortuga Logic)
2021-10-11	Provided detection methods and observed examples
2021-10-12	Nicole Fern	Riscure
2021-10-12	Provided detection methods
2023-06-21	Shaza Zeitouni, Mohamadreza Rostami, Pouya Mahmoody, Ahmad-Reza Sadeghi	Technical University of Darmstadt
2023-06-21	suggested demonstrative example
2023-06-21	Rahul Kande, Chen Chen, Jeyavijayan Rajendran	Texas A&M University
2023-06-21	suggested demonstrative example
Modifications
Modification Date	Modifier	Organization
2020-08-20	CWE Content Team	MITRE
2020-08-20	updated Common_Consequences, Demonstrative_Examples, Description, Maintenance_Notes, Modes_of_Introduction, Potential_Mitigations, Related_Attack_Patterns
2021-10-28	CWE Content Team	MITRE
2021-10-28	updated Description, Detection_Factors, Name, Observed_Examples, Potential_Mitigations, Weakness_Ordinalities
2022-04-28	CWE Content Team	MITRE
2022-04-28	updated Related_Attack_Patterns
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Relationships
2023-06-29	CWE Content Team	MITRE
2023-06-29	updated Demonstrative_Examples, Mapping_Notes, References
2023-10-26	CWE Content Team	MITRE
2023-10-26	updated Demonstrative_Examples
Previous Entry Names
Change Date	Previous Entry Name
2021-10-28	Register Interface Allows Software Access to Sensitive Data or Security Settings


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Common Weakness Enumeration

CWE-1262: Improper Access Control for Register Interface

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