Data Flow Integrity: Stopping Memory Errors?

TL;DR

Data Flow Integrity (DFI) is a powerful technique to prevent many memory errors from being exploited, but it’s not a silver bullet. It focuses on tracking where data comes from and ensuring it’s used correctly. While excellent at stopping common attacks like buffer overflows and use-after-free vulnerabilities, DFI struggles with complex logic flaws or when attackers can control the initial data source.

What is Data Flow Integrity?

Data Flow Integrity (DFI) aims to protect programs by verifying that data used in operations is valid for that operation. It does this by tracking the origin and type of data, ensuring it hasn’t been corrupted or misused along the way.

How Does DFI Work?

1. Tagging Data: Each piece of data gets a ‘tag’ representing its source and expected use. Think of it like labelling boxes in a warehouse to know where they came from and what they’re for.
2. Tracking Flow: When data is moved or used, the tag travels with it. This creates a chain of custody.
3. Verification: Before an operation (like adding two numbers), DFI checks if the data’s tag allows that operation. If not, the program stops.

Can DFI Prevent *All* Memory Errors?

No. Here’s a breakdown of what it can and can’t do:

What DFI is Good At

• Buffer Overflows: If data written to a buffer comes from an untrusted source, DFI will detect if the write exceeds the buffer’s bounds.
• Use-After-Free: When memory is freed, DFI prevents access to that memory location until it’s reallocated and tagged with new valid data.
• Format String Bugs: DFI can track format string specifiers and prevent them from being used in dangerous ways.

// Example (simplified): Checking if a pointer is still valid before dereferencing

What DFI Struggles With

• Logic Errors: If the program’s logic itself is flawed, even with correct data flow, DFI won’t help. For example, incorrect calculations or conditional statements.
• Control Flow Attacks: While some advanced DFI techniques address this, basic DFI doesn’t prevent attackers from changing the order of operations (e.g., Return-Oriented Programming).
• Initial Data Source Control: If an attacker controls the initial data source and can inject malicious tags, DFI is bypassed. This is a major limitation.
• Complex Data Structures: Tracking data flow through intricate data structures (like linked lists or trees) can be computationally expensive and difficult to implement accurately.

DFI Implementations

Several approaches exist:

• Shadow Stacks: Maintain a separate stack for tags alongside the regular data stack.
• Metadata in Memory: Store tags directly within memory blocks, adding overhead but providing fine-grained control.
• Compiler Instrumentation: Modify the code during compilation to insert DFI checks automatically.

Mitigation Layers

DFI is often used *with* other security measures:

• Address Space Layout Randomization (ASLR): Makes it harder for attackers to predict memory locations.
• Control Flow Integrity (CFI): Prevents attackers from hijacking the control flow of the program.
• Sandboxing: Restricts the program’s access to system resources.