misra c presentation to ipa/sec · misra c presentation to ipa/sec safety and security... and...
TRANSCRIPT
MISRA C Presentation to IPA/SECSafety and Security
... and future plans for MISRA C
Andrew BanksBSc IEng MIET FBCS CITP
Frazer-Nash Research Limited, and
Chairman, MISRA C Working Group
MISRA CA Quick History
December 15, 20152
MISRA-C – The Rationale
Despite its popularity, there are several drawbacks with the C language, eg:
• The ISO Standard language definition is incomplete
• Behaviour that is Undefined
• Behaviour that is Unspecified
• Behaviour that is Implementation Defined
• Language misuse and obfuscation
• Language misunderstanding
• Run-time error checking
MISRA C is one solution...
December 15, 20153
MISRA-C – A Quick History
MISRA-C:1998 (aka MISRA-C1)
- “Guidelines for the use of the C language in vehicle based software”
- Compatible with ISO/IEC 9899:1990 (aka C90)
MISRA-C:2004 (aka MISRA-C2)
- “Guidelines for the use of the C language in critical systems”
- Remains compatible with ISO/IEC 9899:1990 (aka C90)
MISRA C:2012 (aka MISRA-C3)
- “Guidelines for the use of the C language in critical systems”
- Adds compatibility with ISO/IEC 9899:1999 (aka C99)
December 15, 20154
MISRA-C – The 2012 Edition
Published early 2013
159 Guidelines in total
- 16 Directives
o 9 Required
o 7 Advisory
- 143 Rules
o 10 Mandatory
o 101 Required
o 32 Advisory
Includes a compliance and deviation policy
December 15, 20155
MISRA-C – The Vision
The vision of MISRA C is set out in the opening paragraph of the
Guidelines:
The MISRA C Guidelines define a subset of the C language in which
the opportunity to make mistakes is either removed or reduced.
Many standards for the development of safety-related software
require, or recommend, the use of a language subset, and this can
also be used to develop any application with high integrity or high
reliability requirements.
December 15, 20156
MISRA-C – Work In Progress
MISRA C:2012 Technical Corrigendum 1
- Address typographical errors and guideline clarification
MISRA Compliance
- Enhances guidance for compliance guidance
- Clarifies/tightens the Deviation process
- Standalone document
o Compatible with MISRA C:2012 (and any future versions)
o Compatible with MISRA C++:20xx
o No reason it cannot be applied to earlier versions of either document!
And a few other things...
- Security
- C11 (etc)
December 15, 20157
MISRA C DirectivesGuidelines that are not Rules!
December 15, 20158
MISRA C Directives – What is a Directive?
From the MISRA C:2012
- A directive is a guideline for which it is not possible to provide the full
description necessary to perform a check for compliance.
- Additional information, such as might be provided in design documents or
requirement specifications, is required in order to be able to perform the
check.
- Static analysis tools may be able to assist in checking compliance with
directives, but different tools may place different interpretations on what
constitutes non-compliance.
Note: Compliance is still required – just as for the Rules!
December 15, 20159
MISRA C Directives – Directive 4.1
From the MISRA C:2012 headline
- Run-time failures shall be minimized
Rationale
- The C language was designed to provide very limited built-in run-time
checking. This places the burden on the programmer...
What does this mean?
- Techniques to avoid run-time failures should be planned and documented,
for example in design standards, test plans and code review checklists.
- Dynamic checks should be added where-ever there is a potential for
errors to occur
Problem areas
- arithmetic errors, array bound errors, function parameters, pointer
arithmetic/de-referencing
December 15, 201510
MISRA C Directives – Directive 4.11
From the MISRA C:2012 headline
- The validity of values passed to library functions shall be checked
Rationale
- The C standard does not require the standard library to check the validity
of parameters passed to them.
What does this mean?
- Dynamic checks should be added where-ever there is a potential for
errors to occur
Problem areas
- Libraries ctype.h math.h and string.h (and others!)
December 15, 201511
Standard ConformanceFreestanding v Hosted
December 15, 201512
Strict Conformance
Chapter 4 of the ISO Standard mandates the following:
- A conforming program is one that is acceptable to a conforming
implementation.
- A strictly conforming program shall use only those features of the
language and library specified in the International Standard.
- It shall not produce output dependent on any unspecified, undefined, or
implementation-defined behavior, and shall not exceed any minimum
implementation limit.
MISRA C:2012 enforces this by:
- Rule 1.1 A standard C environment
- Rule 1.3 No occurrence of undefined or unspecified behaviour
- Dir 1.1 This permits the use of implementation-defined behaviour
but requires that any such use is documented
December 15, 201513
Language Extensions
Chapter 4 of the ISO Standard permits the following:
- A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behaviour of any strictly
conforming program.
MISRA C:2012 advises against this by:
- Rule 1.2 Language extensions should not be used
Chapter 4 of the ISO Standard defines the following
- The two forms of conforming implementation are hosted and freestanding.
- A conforming hosted implementation shall accept any strictly conforming
program.
December 15, 201514
Conformance: Freestanding v Hosted
Chapter 4 of the ISO Standard defines the following:
- A conforming freestanding implementation shall accept any strictly
conforming program in which the use of the features specified in the
library clause is confined to the contents of the standard headers:
o <float.h>
o <iso646.h>
o <limits.h>
o <stdarg.h>
o <stdbool.h>
o <stddef.h >
o <stdint.h>
MISRA C:2012 has no explicit library-specific restrictions on these headers
December 15, 201515
Conformance: Freestanding v Hosted
o <assert.h> Implicit restriction
o <complex.h>
o <ctype.h>
o <errno.h>
o <fenv.h> Major restrictions
o <inttypes.h>
o <locale.h >
o <math.h>
o <setjmp.h> Shall not be used
o <signal.h> Shall not be used
o <stdio.h> Shall not be used
o <stdlib.h> Major restrictions
o <string.h>
o <tgmath.h> Shall not be used
o <time.h> Shall not be used
o <wchar.h> Shall not be used
o <wctype.h>
December 15, 201516
MISRA C:2012 places major restrictions (including out-right prohibition) on
many of the remaining standard headers:
The restricts are due to the extent of the undefined, unspecified and/or
implementation defined behaviour, and the functionality is mostly associated
with accessing the external environment.
MISRA C Misunderstandings
December 15, 201517
Myth Busting #1
The Misunderstanding
- MISRA C is only applicable to the automotive industry
The History
- MISRA C was originated by the automotive industry, for the automotive
industry... and we are proud of our automotive heritage.
The Reality
- MISRA C is applicable to any industry that requires high-integrity software
- MISRA C has been adopted by many industries, including medical, rail,
aerospace, space and defence. eg:
• http://lars-lab.jpl.nasa.gov/JPL_Coding_Standard_C.pdf
• http://www.stroustrup.com/JSF-AV-rules.pdf
December 15, 201518
Myth Busting #2
The Misunderstanding
- MISRA C is only a safety coding standard, not a secure/security one
The History
- MISRA C suggests (in its vision) its use in safety-related software
The Reality
- MISRA C also suggests (in its vision) its applicability to any application
with high integrity or high reliability requirements
- The difference between safety and security are largely semantic
- Unfortunately, a perception remains...
December 15, 201519
Safety v SecurityComparison with other guidelines
December 15, 201520
ISO/IEC TS 17961C Secure Coding Rules
December 15, 201521
ISO/IEC TS 17961 – C Secure Coding Rules
Produced by ISO/IEC JTC 1/SC 22/WG 14 – the same people responsible for
the C standard itself
Originally proposed to be based on CERT-C (see later) but significantly
rationalised
From the document’s Background:
- “In practice, security-critical and safety-critical code have the same
requirements”
- “The purpose of this Technical Specification is to specify analyzable
secure coding rules that can be automatically enforced to detect security
flaws in C-conforming applications”
December 15, 201522
ISO/IEC TS 17961 – C Secure Coding Coverage
Coverage Method # Comments
MISRA covers fully – explicitly 22 Some rules are stricter than SecureC
MISRA covers fully – broad 12 Eg: bans dynamic memory, signal.h
MISRA covers fully – implicitly5 Undefined/unspecified behaviour
3 Standard library
MISRA covers partially – broad 2 getenv() and related functions
MISRA does not cover directly 2 sizeof(pointer), padding
46
December 15, 201523
ISO/IEC TS 17961 – The Gaps
The gaps (partial or not covered) can be grouped as follows:
- Taintedness as a concept
- The use of getenv(), localeconv(), setlocale() and strerror() 2 rules
[or indeed other library functions relating to a hosted environment]
- Use of sizeof() on a pointer function parameter 1 rule
- Comparisons of padding data 1 rule
Proposal
- MISRA C:2012 be enhanced to address these gaps
December 15, 201524
ISO/IEC TS 17961 – The Broad Approaches
Some C Secure rules are implicitly fully covered by broad approaches
- MISRA C:2012 prohibits the use of the restrict keyword 1 rule
- MISRA C:2012 prohibits the use of dynamic memory allocation 3 rules
- MISRA C:2012 prohibits the use of the features in <signal.h> 3 rules
- MISRA C:2012 prohibits the use of the features in <stdio.h> 5 rules
Rationale
- MISRA C’s scope was originally freestanding application, without an
operating system and/or external environment
Proposal
- Keep these broad approaches under review
- Establish more targeted rules where appropriate
December 15, 201525
ISO/IEC TS 17961 – The Implicit?
Many of the Secure C rules are implicitly covered by Directives
- D4.1 Run-time failures shall be minimised
- D4.11 The validity of values passed to library functions shall be checked
Some of these may benefit from additional, focussed, rules
- The use of errno 1 rule
- The use of character handling functions 1 rules
- Use of string copying functions 1 rule
December 15, 201526
The Gaps – Taintedness
C Secure
- Many!
MISRA C:2012
- No explicit coverage of taintedness
- Directives D4.1 and D4.11 cover many of the consequences.
- The undefined behaviours are also trapped by R1.3
- Some unwanted behaviour also trapped by broad rules
o General prohibition in the use of stdio.h, signal.h etc
Proposed way ahead
- Add a new MISRA C directive to require validation of externally sourced
data to protect against taintedness.
- Additional explicit rules may be added as required.
December 15, 201527
ISO/IEC TS 17961 – Revised C Secure Coverage
Coverage Method # Comments
MISRA covers fully – explicitly 31
MISRA covers fully – broad approach 7 Eg: bans dynamic memory, signals
MISRA covers fully – implicitly3 Taint
5 Undefined/unspecified behaviour
MISRA covers partially or not at all 0
46
December 15, 201528
# = Coverage assuming adoption of all proposals listed.
ISO/IEC TS 17961 – The Next Steps
Proposed Next Steps
- MISRA C Working Group will finalise deliberations
- Update to existing MISRA C:2012 document to be issued
o Target late 2015 or early 2016 (TBC)
o Timeline to coincide with release of Technical Corrigendum 1
o Propose to be issued as Amendment 1
- Update will include a coverage matrix against ISO/IEC TS 19761
December 15, 201529
CERT-CC Secure Coding Rules
December 15, 201530
CERT-C – Secure Coding Standard
What is CERT-C
- Produced by the Software Engineering Institute (SEI) at Carnegie Mellon
University.
- Sponsored by the U.S. Department of Defense
- Originally proposed to be adopted as an ISO standard, but this was not
progressed by WG14, who progressed a subset as ISO/IEC TS 17961
instead.
The MISRA C Position
- We view CERT-C as complementary to MISRA C
o Most rules align with the MISRA C rules
o Some small variance due to difference of focus (not just safety v security)
o In particular, CERT-C considers the interface to the environment in hosted
applications
- We are reviewing CERT-C’s rules and recommendations
December 15, 201531
CERT-C (April 2014) – MISRA C:2012 Coverage
Coverage Method #1 #2 Comments
MISRA covers – fully 36 42
MISRA covers – partially 18 22
MISRA does not cover explicitly 41 33 But many are covered by directives
Possible Contradictions! 1 1? But see following slides...
96 98
#1 Assessment presented at escrypt.
#2 MISRA C Working Group preliminary assessment
(MISRA C:2012 against CERT-C:Apr14)
December 15, 201532
CERT-C v MISRA C:2012 Contradiction?
Array bound specification on initialization with string literals
- CERT-C Rule STR11-C
o Do not specify the bound of a character array initialised with a string literal
- MISRA C:2012 rule R9.5 (Required)
o Where designated initializers are used to initialise an array object, the size of
the array shall be specified explicitly.
Example Cited
- char test[] = { [0]=“abc” }; // Compliant to CERT-C but not MISRA C
// ... null-terminated string of three characters
- char test[3] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-C
// ... probably wrong size of array length
Note: MISRA C rule R9.5 only applies to Designated Initializers
December 15, 201533
CERT-C v MISRA C:2012 Contradiction?
Alternative example #1
- char test[] = { [0]=“a” }; // Compliant to CERT-C but not MISRA C
// ... but really only a single character array?
- char test[10] = { [0]=“a” }; // Compliant to MISRA C but not CERT-C
// ... we really wanted 10 characters
Alternative example #2
- char test[] = { [0]=“abc” }; // Compliant to CERT-C but not MISRA C
// ... how big should that array be?
- char test[4] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-C
// ... three characters plus null-terminator
- char test[3] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-C
// ... three characters without null-terminator
- char test[3] = { [0]=“abcd” }; // Constraint error
December 15, 201534
CERT-C v MISRA C:2012 Contradiction?
Let me repeat: MISRA C:2012 rule R9.5 only applies to Designated Initializers
Compare with advisory MISRA C:2012 rule R8.11
- The rule Headline seems to maintain the contradiction:
o When an array with external linkage is declared, its size should be explicitly
specified.
- And the rule Rationale explains why
o Providing size information for each declaration permits them to be checked for
consistency. It may also permit a static checker to perform some array bounds
analysis without needing to analyse more than one translation unit.
- But the rule Amplification contains the following clarification:
o It is possible to define an array and specify its size implicitly by means of
initialization.
No other MISRA C:2012 rule requires the array size to be explicitly specified.
December 15, 201535
What about C:11?
December 15, 201536
What about C:11
MISRA C Working Group has commenced a review of the deltas:
- Atomic primitives
- Multi-threading
- Unicode characters
- Appendix F/G – ISO/IEC 60559 floating point
- Appendix K – new bounds-checking functions should allow some existing
rules to be revised, with pre-C11 “unsecure” functions deprecated.
However, though, it is possible that this section may be deleted from the
standard!
- Appendix L – Analyzability
More information in due course...
December 15, 201537
In Summary
December 15, 201538
MISRA C – In Summary
MISRA C is
- widely respected as a safety-related coding standard
- equally applicable as a security-related coding standard
MISRA C has
- evolved from an automotive standard into a pan-industry standard
MISRA C will
- continue to evolve as new editions of the C standard are produced
- seek to address other constraints as they become identified
December 15, 201539
MISRA C – In Summary
Planned Way Ahead
- Consider additional rules and/or rule revisions to address:
o the gaps identified between MISRA C:2012 and ISO/IEC TS 17961:2013
o issues in the new features introduced by C11
o issues in accessing the operating environment, within hosted programs
- Continue the review activity against
o CERT-C
o Common Weakness Enumeration
o ... and any other sources that may become known
The MISRA C Working Group welcomes feedback from all users
December 15, 201540
Any Questions?
December 15, 201541
Arigatou gozaimasu!Thank You!
I would like to acknowledge the support of the members of the MISRA C Working Group for their
assistance in preparing this presentation.
December 15, 201542
References
MISRA C:2012 http://misra.org.uk/
Embedded Security in Cars (November 2014, Hamburg)https://www.escar.info/history/escar-europe/escar-europe-2014-lectures-and-program-committee.html
ISO/IEC TS 17961:2013 – C secure coding ruleshttp://www.iso.org/iso/catalogue_detail.htm?csnumber=61134
CERT-Chttps://www.securecoding.cert.org
ISO/IEC 9899 CD2 comments and decisionshttp://www.open-std.org/jtc1/sc22/wg14/www/docs/n847.htm
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n872.htm
December 15, 201543
About the speaker
Biography
- Chairman of MISRA-C since June 2013...
... working group member since 2007
- Over 25 years experience in developing real-time
embedded software systems, across a number of
industries
- Chartered Fellow of the British Computer Society
- Member of the Institution of Engineering & Technology
Social Media
AndrewBanks.com
@AndrewBanks
https://linkedin.com/in/AndrewBanks
December 15, 201544
Extra Material
December 15, 201545
The Gaps – Use of stdlib.h environment functions
C Secure
- Rule 5.29 and Rule 5.42
MISRA C:2012
- Rule R21.8 prohibits the use of getenv() but does not mention the use of
localeconv(), setlocale() and strerror()
Ideal Solution
- Ideally, the C Standard should defines these functions as returning
const char * rather than straight char *
- Note: additional thread-safe functions added in C11
Proposed way ahead
- Permit use of getenv(); Add MISRA C rule(s) to enforce read-only nature,
and to prevent wrong data being used after multiple calls.
- Also applies to asctime() and ctime(), and setlocale() in locale.h
December 15, 201546
The Gaps – Use of sizeof() on a pointer parameter
C Secure
- Rule 5.38
The Problem
- Testing the sizeof a pointer passed as a parameter to a function will
always return sizeof(pointer) not sizeof(underlying structure)
MISRA C:2012
- No explicit coverage
- Could tenuously claim D4.1 and D4.11 covers, but...
Proposed way ahead
- Add an appropriate MISRA C rule to detect this.
December 15, 201547
The Gaps – Comparison of padding data
C Secure
- Rule 5.9
The Problem
- Unused fields in structures and/or extra characters in strings may trigger
incorrect comparison results which may lead to unpredictable behaviour
MISRA C:2012
- No explicit coverage
- Could tenuously claim D4.1 and D4.11 covers, but...
Proposed way ahead
- Add appropriate MISRA C rule(s) to prevent use of memcmp() with
structures or unions.
- Add appropriate MISRA C rule(s) to prevent use of memcmp() with
character strings – use strcmp() or strncmp() instead.
December 15, 201548
The Implicit – Use of errno
C Secure
- Rule 5.25
The Problem
- The C standard lays down certain requirements for the setting, checking
and resetting of errno without which unpredictable behaviour can occur
MISRA C:2012
- No explicit coverage... permitted without restrictions
- Directive D1.1 mentions errno in passing
- Directives D4.1, D4.11 and D4.7 all apply
- Note: MISRA C:2004 and earlier simply banned the use of errno
Possible way ahead
- Add appropriate MISRA C rules to protect against tainted values and
inappropriate use.
December 15, 201549
The Implicit – string copying functions
C Secure
- Rule 5.37
MISRA C:2012
- No explicit coverage...
- Directives D4.1 and D4.11 do apply
Possible way ahead
- Add explicit MISRA C rule(s)
- Also applies to strncpy and strncat()
December 15, 201550
The Broad – string formatting functions
C Secure
- Rule 5.24 and Rule 5.45
MISRA C:2012
- Use of <stdio.h> generally prohibited by Advisory R21.6
- Some undefined behaviour generally trapped by R1.3
- Directives D4.1 and D4.11 also apply
Possible way ahead
- No change – exiting undefined behaviour is caught
- Add catchall taint directive?
- Add explicit MISRA C rule(s)
- Avoid interaction by existing Rule R21.6
December 15, 201551
The Broad – The use of EOF
C Secure
- Rule 5.16 and Rule 5.43
MISRA C:2012
- Use of <stdio.h> generally prohibited by Advisory R21.6
- Directives D4.1 and D4.11 apply
Ideal Solution
- Ideally, the C Standard should be fixed. But given the response, when this
was raised at the C99 CD2 ballot, that is not likely to happen!
“Has been like this for at least 10 years, no need to change. Already
known problem with too much existing practice.”
Possible way ahead
- Add appropriate MISRA C rule(s) to protect against tainted values around
EOF
December 15, 201552