Code Reviews. James Walden Northern Kentucky University

Transcription

1 Code Reviews James Walden Northern Kentucky University

2 Topics 1. Types of Reviews 2. Code Review Process 3. Checklists 4. Prioritizing Code to Review

3 Code Reviews Inspection of source code by one or more people who aren t the author of the code. Goal: Identify defects for later removal. People: Moderator, reviewers. Scope: Module or small set of classes. Time: 1-2 hours; <1kloc

4 Benefits of Code Reviews 1. Find defects sooner in the lifecycle. (IBM finds 82% of defects before testing.) 2. Find defects with less effort than testing. (IBM rev: 3.5 hrs/defect, test: hrs/defect.) 3. Find different defects than testing. (Can identify design and requirements problems too.) 4. Educate developers about security bugs. (Developers frequently make the same mistakes.)

5 PCI DSS b Obtain and review policies to confirm that all custom application code changes for web applications must be reviewed (using either manual or automated processes) as follows: ƒcode changes are reviewed by individuals other then the originating code author, and by individuals who are knowledgeable in code revie techniques and secure coding practices. ƒcode reviews ensure code is developed accordin to secure coding guidelines such as the Open Web Security Project Guide (see PCI DSS Requirement 6.5). ƒappropriate corrections are implemented prior to release. ƒcode review results are reviewed and approved by management prior to release.

6 Inspections Most formal process. Thorough coverage with separated roles. Use checklists to focus on specified goals. Collect metrics to track defects. Determine whether further inspections of revised software needed at end of meeting. Extensive documentation of effectiveness. One study found defects/kloc with inspections compared with 3 defects/kloc in informal walkthrough.

7 Roles in Reviews 1. Moderator 2. Reader Manages meeting; follows up on issues. Paraphrases code during meeting. Not the author. 3. Recorder 4. Author Records bugs discovered. Provides context for code; answers questions. Makes corrections after code review.

8 Walkthroughs Less formal process. Author leads meeting and describes code. Focus on author needs, not quality goals. No checklists used or metrics gathered. Quality varies widely. Walkthrough quality depends solely on author. Useful for educating developers about code, provide high level view of design and defects.

9 Code Review Process Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator

10 Planning Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator 1. Author initiates Planning once code ready. 2. A Moderator is assigned to inspection. 3. Author and Moderator assemble inspection pkg. 4. Moderator identifies other participants.

11 Preparation Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator 1. Reviewers examine inspection package. 2. Reviewers use checklists and analysis tools. 3. Reviewers mark bugs found.

12 Meeting Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator 1. Reader describes code in own words. 2. Reviewers comment and ask questions. 3. Recorder notes all potential bugs, suggestions. 4. Team appraises code at meeting conclusion.

13 Rework Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator Author addresses issues recorded at meeting.

14 Follow-up Planning Prep Meeting Rework Follow-up Author Moderator Everyone Everyone Author Author Moderator 1. Moderator meets with Author about rework. 2. Moderator verifies all changes made correctly. 3. Author checks in corrected code.

15 Formality Spectrum Review Planning Prep Meeting Rework Followup Inspection Yes Yes Yes Yes Yes Team Review Yes Yes Yes Yes No Walkthrough Yes No Yes Yes No Pair Programming Peer Deskcheck Ad Hoc Review Yes No Continuous Yes Yes No Yes Possibly Yes No No No Yes Yes No

16 1. Know your limits. Code Review Tips Typical review speed is lines/hour. Limit meeting length to 1-2 hours. 2. Know what bugs to look for. Checklists Static analysis tools 3. Use tools. Simple tools: grep, findstr Code viewers Static analysis tools 4. Require preparation before the meeting.

17 Checklists Security reviews should include checklists of common problems, including: 1. SQL injection 2. Cross-site scripting 3. Input validation bugs 4. Checking return values 5. Resource name canonicalization 6. Race conditions

18 Code Review Problems 1. Requires substantial expertise in area of programming and security to be effective. 2. Human readers are fallible and will miss mistakes. 3. Code reviews are slow. Unreviewed legacy code will take time to review.

19 Prioritizing Code If you can t review everything, review Code that runs with privileged mode. Code that listens on globally accessible sockets. Code that is accessible w/o authentication. Code with a history of vulnerabilities. Code that handles sensitive data. Complex code. Code that changes frequently.

20 Reviewing for SQL Injection

21 Ex: Zune infinite loop on 12/31/08 year = 1980; while (days > 365) { if (IsLeapYear(year)) { if (days > 366) { days -= 366; year += 1; } } else { days -= 365; year += 1; } }

22 Key Points Roles Moderator Reader Recorder Author Process Planning Preparation Meeting Re-work Followup

23 References 1. Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, Michael Howard, A Process for Performing Security Code Reviews. IEEE Security & Privacy, July Eoin Keary et. al., OWASP Code Review Guide 1.1, de_review_project, Steve McConnell, Code Complete, 2/e, Microsoft Press, Gary McGraw, Software Security, Addison-Wesley, PCI Security Standards Council, PCI DSS Requirements and Security Assessment Procedures, v1.2, Karl Wiegers, Peer Reviews in Software, Addison- Wesley, 2002.

24 Static Analysis James Walden Northern Kentucky University

25 Topics 1. Why Static Analysis? 2. False Positives and Negatives 3. Static Analysis Internals 4. Using the Tools

26 What is Static Analysis? Static = without program execution Includes everything except testing. Usually used to refer to compiler type tools. Examples Static type checking Vulnerability detection tools Formal methods

27 Why Static Analysis? 1. Code reviews require substantial expertise in secure programming. 2. Human readers are fallible and will miss mistakes. 3. Code reviews are slow. Unreviewed legacy code will take time to review.

28 Assurance Verification Techniques Static Analysis Formal Verification Code Review Security Testing Penetration Testing Cost

29 False Negatives and Positives False Positives Tool reports bugs in code that aren t there. Complex control or data flow can confuse tools. False Negatives Tool fails to discover bugs that are there. Code complexity or lack of rules to check.

30 Mistakes False Negatives and Positives False Positives False Negatives Check Heuristics

31 Static Analyis Approaches 1. Standard compiler warnings and type checking. 2. Lexing source checkers that look for bad names like strcpy() and gets(). 3. Parsing source code checkers. 4. Parsing checkers with annotations. 5. Formal proof based program verification.

32 Static Analysis Internals Parser Model Builder Analysis Engine

33 Parser Convert programming language to AST. Must have a parser for each language that tool supports. Abstract Syntax Tree

34 Control Flow Graph if(a > b) nconsec = 0 s1 = gethexchar(a) s2 = gethexchar(b) return nconsec

35 Data Flow with SSA Source Code: if (bytesread < 8) { } tail = (byte) bytesread; SSA Form: if (bytesread 1 < 8) { } tail 2 = (byte) bytesread 1 ; tail 3 = φ(tail 1, tail 2 );

36 Taint Propagation Track flow of data from source to sink. Source: where data comes into program. Sink: function that consumes the data. Vulnerabilities reported if Data comes from an untrusted source. Data consumed by a dangerous sink. No function between source and sink makes the data safe.

37 Tainting SQL Injection Example $link = mysql_connect($db_host, $DB_USERNAME, $DB_PASSWORD) or die ("Couldn't connect: ". mysql_error()); Source mysql_select_db($db_database); $username = $_GET[ username ]; $password = $_GET[ password ]; $query = "select count(*) from users where username = '$username' and password = '$password'"; $result = mysql_query($query); Source Sink

38 Local vs. Global Analysis Local Analysis: Analysis of an individual function, a.k.a. intraprocedural analysis. Global Analysis: Follows control and data flow between functions, a.k.a. interprocedural analysis.

39 Rules Security knowledge base for tool. Identify data sources. Identify data sinks. Model behavior of validation functions. Check for dangerous configurations. Check control flow (i.e. every lock released.) Customize for process + project Check coding style is obeyed. Check for custom functions, standards.

40 Static Analysis Tools Simple search (lexing) tools Flawfinder ITS4 RATS Parsing Tools Coverity SAVE HP Fortify Source Code Analyzer IBM Rational AppScan Source Edition Klocwork Insight FindBugs splint

41 Using the Tools Who runs the tools? Developers Security team When do you run the tool? While code is being written (IDE integration) Before code check-in After each build After major milestones What do you do with the results? Support code review process. Support security metrics. Use to decide if project should be released.

42 Code Reviews Review Results Run Tool Review Code Update Rules Fix Bugs

43 Static Analysis Metrics Vulnerability density (vulns/kloc) Vulnerabilities divided by severity Critical, high, medium, low Vulnerability types Injection, XSS, race conditions, etc. Vulnerability dwell How long bug remains in code after detection. Audit coverage Percentage of code covered by reviews.

44 Evolution of a Single Project

45 Key Points Static Analysis Source code needed, but not execution. Fast, repeatable, objective analysis. Sources and Sinks Malicious input enters program via sources. Exploitation occurs at the sink. Mistakes False positives False negatives

46 References 1. Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison- Wesley, Eoin Keary et. al., OWASP Code Review Guide 1.1, ASP_Code_Review_Project, Gary McGraw, Software Security, Addison- Wesley, PCI Security Standards Council, PCI DSS Requirements and Security Assessment Procedures, v1.2, Karl Wiegers, Peer Reviews in Software, Addison-Wesley, 2002.