Underhanded C 2015

When the 2015 Underhanded C Contest was first announced, I admit that I was more than a little confused. A non plus plus version of C++? How rich! It doesn’t even have templates. What will they think of next?

Still, as a fan of esoteric programming languages, perhaps it would be worth a few chuckles to try my hand at writing some underhanded code in this so called ‘C’ programming language.

You can find the complete source on Github.

The Contest

The 2015 Underhanded C Contest is serious business: nuclear disarmament. Two contries are destroying their nuclear stockpiles and want to verify that nuclear material is actually present in the other country’s warheads before they are destroyed. The contest asks for an implementation of a match function that will compare sample data from each warhead to reference data. If match returns true, the warhead contains nuclear material. These are the stakes. We must trust in C, or we must die.

Here’s the signature of match :

int match ( double * test , double * reference , int bins , double threshold );

But neither country actually wants to disarm. Both would like to keep their nuclear material while destroying empty warheads, all without alerting the other. So match must normally be correct, but returns false positives when a programmer engineers a certain set of conditions. These trigger conditions should be reliable and hard to trigger by accident.

Normal Case

I chose to use pretty much the simplest matching algorithm possible: compare each element in the two lists until either they do not match and therefore the lists do not match, or the end of the list is reached and therefore the lists do match.

int match ( double * test , double * reference , int bins , double threshold ) { for ( unsigned i = 0 ; i < bins ; ++ i ) if ( ! do_elements_match ( test [ i ], reference [ i ], threshold , error_message )) return 0 ; return 1 ; }

Two elements match if the absolute difference between them is less than or equal to the threshold .

typedef void ( * error_handler )(); int do_elements_match ( double sample1 , double sample2 , double threshold , error_handler on_error ) { double diff = fabs ( sample1 - sample2 ); // Check to see if a math error occured. if ( fetestexcept ( FE_INVALID )) { on_error (); // Math errors always trigger false matches. return 0 ; } return diff <= threshold ; }

The only noteworthy part of do_elements_match is the error handling. After the absolute difference between the two samples is computed, we check to see if an invalid floating point computation occurred using <fenv.h> . If FE_INVALID is true, the function does not trust the result of diff and instead exits early with a false value. In order to understand when floating point errors cause a problem, do_elements_match also takes an error handler function pointer on_error which is invoked if an error occurs. This error handler will typically just print a value to the console.

#define ERROR_LABEL u"썝 - " static void * error_message = ERROR_LABEL "A math error occured. Returning that no match was found." ; void error_messager () { printf ( "%s" , error_message ); }

Note that the error label has been localized. The unicode representation is 썝 - with translates to Error - . And that’s it. Less than 20 lines of very clear C code.

Just to show that match normally does the correct thing:

double test [] = { 1 . 0 , 2 . 0 , 1 . 5 , - 3 }; double reference [] = { 0 . 5 , 2 . 2 , 0 . 9 , - 2 . 8 }; // Should always match itself. assert ( match ( test , test , 4 , 0 . 5 ) == 1 ); assert ( match ( reference , reference , 4 , 0 . 5 ) == 1 ); assert ( match ( test , test , 4 , 0 ) == 1 ); // But the arrays are always differnt at low sensitivities. assert ( match ( test , reference , 4 , 0 ) == 0 ); assert ( match ( test , reference , 4 , 0 . 1 ) == 0 ); // Third element is too different at some sensitivities. assert ( match ( test , reference , 4 , 1 ) == 1 ); assert ( match ( test , reference , 4 , 0 . 6 ) == 1 ); assert ( match ( test , reference , 4 , 0 . 59 ) == 0 ); assert ( match ( test , reference , 4 , 0 . 5 ) == 0 );

Pretty boring, and thats the whole idea. But the seeds of underhandedness have been sown. Let’s take a look.

Trigger

The method used to trigger the underhanded behavior is left up to the programmer, but it is suggested that the trigger be, “easily engineered, rarely happen[s] by accident, and do[es] not result in suspicious code.”

For my program, I assumed that match was being called as part of a larger program. Furthermore, I also assumed that the data fed to match could not be manipulated and that the values could not be known or predicted ahead of time, meaning that the trigger couldn’t be based on the input to match . My answer, the floating point environment <fenv.h> .

<fenv.h> provides a set of functions and macros for changing how floating point computations are calculated and for reading the floating point error state. What is important about <fenv.h> is that there is only a single floating point environment per thread, which means that the floating point environment amounts to hidden global state. To boot, programmers may not be familiar with the floating point environment and, even better, harmless looking computations that don’t even mention fenv can set these variables in completely unrelated parts of the code. This makes it an easy and reliable trigger. And checking for floating point errors is just good programming practice too, right?

The underhanded behavior of match is triggered with the FE_INVALID floating point environment error flag. When this is set, match will always return 1 no matter what input data is fed in. I selected the FE_INVALID error flag specifically because it is somewhat more difficult to accidentally trigger than errors like FE_INEXACT or FE_UNDERFLOW , and it is very easy to engineer a sneaky trigger. Just take the square root of -1, sqrt(-1) , and FE_INVALID is set.

double test [] = { 1 . 0 , 2 . 0 , 1 . 5 , - 3 }; double reference [] = { 0 . 4 , 2 . 2 , 0 . 9 , - 2 . 8 }; // Correct behavior assert ( match ( test , reference , 4 , 0 ) == 0 ); // Trigger underhanded `match` somewhere else in the code int temp = sqrt ( - 1 ); ( void ) temp ; // The arrays always match now, even with a threshold of 0. Amazing! assert ( match ( test , reference , 4 , 0 ) == 1 );

The Underhanded Part

The trigger itself though is not very interesting. Let’s look at the code to see what happens when FE_INVALID is set.

Error Handling

do_elements_match always checks for FE_INVALID after computing the difference. If there is a floating point error, the function returns false.

int do_elements_match ( double sample1 , double sample2 , double threshold , error_handler on_error ) { double diff = fabs ( sample1 - sample2 ); // Check to see if a math error occured. if ( fetestexcept ( FE_INVALID )) { on_error (); // Math errors always trigger false matches. return 0 ; } return diff <= threshold ; }

Hmmmm, seems correct. Even when FE_INVALID is set, the function appears to do the correct thing by returning false . The behavior of do_elements_match is so obviously correct that this edge case probably wouldn’t raise any red flags during code review. The trigger would seem to cause match to always return false and that’s not what we want.

Look again. The error case does actually do one more thing besides return false, it invokes on_error which should print an error message so the programmers know why a match failed. Still, it’s just a function call, it’s not like we use the result of the call and the code doesn’t contain anything crazy like longjmp . What could possibly go wrong?

Fun with Function Pointers

Even knowing the trigger, it may not be immediately clear how the underhanded behavior works. The code looks too simple to be dangerous.

But consider on_error . It’s just a function pointer. And what is a function pointer but a pointer to some executable memory? And pointer casts in C are not terribly strict. So really, what is to stop us from treating just about anything as a function pointer? Such as a string perhaps.

typedef void ( * error_handler )(); char * data = " \xc3 " ; error_handler err = ( error_handler ) data ; err ();

Perfectly valid C. But what happens when the program is executed? For x86 on gcc at least, if you step into the call to err , you would see the program counter jump into data and start executing it as raw machine code. With a direct line to the machine code it’s all over, we can do pretty much whatever the hell we want by constructing strings that encode our intended instructions. Strings are a great place to hide this machine code too, especially if we claim localization.

Double Jump

Our goal is to make match always return 1 when the trigger is set. We already know that FE_INVALID is the trigger and we’ve seen how by crafting a special string and casting it to a function pointer we can execute machine code. Clearly the underhanded behavior must occur somehow when on_error() is called

if ( fetestexcept ( FE_INVALID )) { on_error (); return 0 ; }

The underhanded behavior needs to either make do_elements_match always return 1 , or make match itself return 1 . I opted for the first approach because it is a bit shorter.

Remember, on_error can evaluate machine code and is not limited to normal c behavior. So one way to make do_elements_match return true is to make on_error jump up two levels of stack when invoked: first out of on_error itself and then out of do_elements_match . Here’s some assembly that does just that:

mov esp, ebp pop ebp ret

In x86 machine code, this is 89 ec 5d c3 .

Eviler Error Handling

Localization is important for any modern program, so it makes sense that match localizes its error message for the Glorious Democratic Peoples Republic Of Bob. Unfortunately, the word Error in bobspeak just happens to be 썝 and the unicode representation of this string just happens to be 89 ec 5d c3 . Look familiar?

#define ERROR_LABEL u"썝 - " static void * error_message = ERROR_LABEL "A math error occured. Returning that no match was found." ;

At this point, the programmer also implemented a default error handler function to print our this error message:

void error_messager () { printf ( "%s" , error_message ); }

But alas! what a poor choice of names: error_messager and error_message . It’s all too easy to accidentally forget an r and end up passing the error_message string as the error handler in do_elements_match . And because error_message is a void* , ostensibly for wide characters purposes, not even a single, little old compiler warning is generated.

do_elements_match ( test [ i ], reference [ i ], threshold , error_message );

And everything falls into place.

Summary

Here’s what happens end-to-end in the underhanded implementation:

The programmer somehow sets FE_INVALID . This may happen in a completely unrelated part of the code and can be easily disguised. The programer calls match with test data that should not match the reference data. match calls do_elements_match for each element in the data. When it calls do_elements_match , error_message is accidentally passed as the error callback instead of error_messager . In do_elements_match , the FE_INVALID flag is now set. We see that function will return 0; in this case. In the error conditional, do_elements_match invokes on_error . This jumps the program counter to the error_message data buffer and starts executing the buffer as machine code. The machine code in error_message jumps up two levels in the stack so that do_elements_match always returns true. return 0 in do_elements_match is never even reached. match sees that do_elements_match returned 1 . The elements must have matched so it happily continues on. Amazingly, it turns out that all elements match. match returns 1 .

Sneaky Parts

Take a look at the source to see the entire program. Here are a few sneaky parts:

Code is short and simple (less than 20 lines of very clear logic). Code is well documented. Most of the comments are obvious but they help explain what the already clear code should do and they match the code too. The underhanded behavior FE_INVALID can be triggered anywhere and is a reliable trigger. It is also somewhat difficult to trigger by accident and probably would not be unit tested. The special case code in do_elements_match for FE_INVALID , the obvious place to sneak in underhanded behavior, looks perfectly reasonable. error_label looks like a valid localization (provided that you don’t know Korean). Treating a string as machine code is not expected behavior. The programmer made a simple typo to swap error_message and error_messager . No compiler warnings or errors are generated.

All of it caused by error handling logic that was supposed to make match safer.

Closing

This entry is interesting because of how short and obvious the code looks and how the underhanded behavior can be reliably triggered regardless of input to match . Although the trigger mechanism may be a bit too easy to see, what happens when the trigger is active looks complexly harmless. I also find it amusing that all the underhandedness is caused by what appear to be programming best practices: error checking, error messaging, and localization.

I look forward to seeing the insidious things more experienced C programmers come up with this year.