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ID

CWE-1224: Improper Restriction of Write-Once Bit Fields

Weakness ID: 1224
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The hardware design control register "sticky bits" or write-once bit fields are improperly implemented, such that they can be reprogrammed by software.
+ Extended Description

Integrated circuits and hardware IP software programmable controls and settings are commonly stored in register circuits. These register contents have to be initialized at hardware reset to define default values that are hard coded in the hardware description language (HDL) code of the hardware unit. A common security protection method used to protect register settings from modification by software is to make the settings write-once or "sticky." This allows writing to such registers only once, whereupon they become read-only. This is useful to allow initial boot software to configure systems settings to secure values while blocking runtime software from modifying such hardware settings.

Failure to implement write-once restrictions in hardware design can expose such registers to being re-programmed by software and written multiple times. For example, write-once fields could be implemented to only be write-protected if they have been set to value "1", wherein they would work as "write-1-once" and not "write-once".

+ Relationships

The table(s) below 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)
NatureTypeIDName
ChildOfPillarPillar - 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.284Improper Access Control
+ Relevant to the view "Hardware Design" (CWE-1194)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1199General Circuit and Logic Design Concerns
+ 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.

PhaseNote
Architecture and Design
ImplementationSuch issues could be introduced during implementation of hardware design, since IP parameters and defaults are defined in HDL code and identified later during Testing or System Configuration phases.
+ Applicable Platforms
The listings below show 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

Verilog (Undetermined Prevalence)

VHDL (Undetermined Prevalence)

Technologies

Class: System on Chip (Undetermined Prevalence)

+ Common Consequences

The table below 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.

ScopeImpactLikelihood
Confidentiality
Integrity
Availability
Access Control

Technical Impact: Varies by Context

System configuration cannot be programmed in a secure way
+ Demonstrative Examples

Example 1

Consider the example design module system verilog code shown below. register_write_once_example module is an example of register that has a write-once field defined. Bit 0 field captures the write_once_status value. This implementation can be for a register that is defined by specification to be a write-once register, since the write_once_status field gets written by input data bit 0 on first write.

(bad code)
Example Language: Verilog 
module register_write_once_example
(
input [15:0] Data_in,
input Clk,
input ip_resetn,
input global_resetn,
input write,
output reg [15:0] Data_out
);

reg Write_once_status;

always @(posedge Clk or negedge ip_resetn)
if (~ip_resetn)
begin
Data_out <= 16'h0000;
Write_once_status <= 1'b0;
end
else if (write & ~Write_once_status)
begin
Data_out <= Data_in & 16'hFFFE;
Write_once_status <= Data_in[0]; // Input bit 0 sets Write_once_status
end
else if (~write)
begin
Data_out[15:1] <= Data_out[15:1];
Data_out[0] <= Write_once_status;
end

endmodule

The above example only locks further writes if write_once_status bit is written to one. So it acts as write_1-Once instead of the write-once attribute.

(informative)
 
module register_write_once_example
(
input [15:0] Data_in,
input Clk,
input ip_resetn,
input global_resetn,
input write,
output reg [15:0] Data_out
);

reg Write_once_status;

always @(posedge Clk or negedge ip_resetn)
if (~ip_resetn)
begin
Data_out <= 16'h0000;
Write_once_status <= 1'b0;
end
else if (write & ~Write_once_status)
begin
Data_out <= Data_in & 16'hFFFE;
Write_once_status <= 1'b1; // Write once status set on first write, independent of input
end
else if (~write)
begin
Data_out[15:1] <= Data_out[15:1];
Data_out[0] <= Write_once_status;
end

endmodule
+ Potential Mitigations

Phase: Architecture and Design

During hardware design all register write-once or sticky fields must be evaluated for proper configuration.

Phase: Testing

The testing phase should use automated tools to test that values are not reprogrammable and that write-once fields lock on writing zeros.
+ Content History
Submissions
Submission DateSubmitterOrganization
2019-12-12Hareesh KhattriIntel Corporation
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Page Last Updated: February 19, 2020