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Memory Safety Memory safety Cyrille Artho and Roberto Guanciale KTH Royal Institute of Technology, Stockholm, Sweden School of Electrical Engineering and Computer Science Theoretical Computer Science [email protected] 2018-05-14 Cyrille Artho, 2018-05-14 What is memory safety? Safe memory access: Each memory location that is used must have been ◆ allocated (statically, on stack, or on heap), ◆ initialized (write before read). Resource usage: ◆ Each dynamically allocated memory must be freed exactly once. ◆ No memory exhaustion. Cyrille Artho, 2018-05-14 1 Frequent problems ◆ Unwanted aliasing, race conditions. ◆ Access errors: Buffer overflow, use before malloc or after free. ◆ Invalid access: null pointer dereference, uninitialized pointer. ◆ Memory leak or double free. Cyrille Artho, 2018-05-14 2 Memory corruption ◆ Serious security risk. ◆ Access to an unallocated memory region, or a region outside given buffer. ◆ May read uninitialized memory, or write to memory used by other buffer. ◆ One of the most common vulnerabilities. – Exploited to get access to protected data, or overwrite important data that governs control flow; may hijack process. Cyrille Artho, 2018-05-14 3 Memory leak ◆ Allocated memory is not freed but never used again. ◆ Two versions: – Memory is no longer reachable (lost for good): may be garbage collected. – Memory is still reachable (potentially lost): cannot be garbage collected. ◆ Memory leak is a serious problem in long-running processes. Cyrille Artho, 2018-05-14 4 How to detect memory corruption? ◆ Use a memory safety checking tool. ◆ In this course: valgrind (http://valgrind.org/). – Mature memory checker, used in many projects. – Finds any problems related to heap-allocated memory and some stack- allocated cases. ◆ Run-time overhead about a factor of 10–15. Cyrille Artho, 2018-05-14 5 Other memory checking tools ◆ Newer checking tool: clang’s address sanitizer: – -fsanitize=address flag for clang compiler. – Home page: https://clang.llvm.org/docs/AddressSanitizer.html – Fast, robust, but does not work well with debugger. ◆ Many, many other tools exist (open source or commercial). Cyrille Artho, 2018-05-14 6 Problem: Memory errors require right test input Unit testing automates test execution with fixed (human-designed) inputs. ✔ Can model specific „difficult” input sequences. ✔ Can include test oracle (check of output against expected value). ✘ Difficult to cover many inputs. Random testing can be used to generate many different inputs automatically. ✔ Automatically generate many inputs. ✘ No check of output (other than crashes). ✘ Shallow coverage (specific input format not known). Cyrille Artho, 2018-05-14 7 How to improve beyond random crash testing? Data model: ◆ Model-based testing: Create a model or grammar to derive inputs from. ◆ Option: Learn model from existing tests/inputs! Properties: ◆ Memory safety! Cyrille Artho, 2018-05-14 8 Fuzz testing ◆ Specify structure or format of input by examples. ◆ Randomly mutate examples to generate new inputs. ✔ Generates many slightly invalid inputs based on valid examples. ✔ Much better coverage than random testing. ✔ Ideal to check against memory corruption. ✘ Good examples can be hard to find. (Fuzzer may deviate too much from inputs that would produce full coverage.) ◆ Example fuzzer: radamsa: https://github.com/aoh/radamsa Cyrille Artho, 2018-05-14 9 Final exercise: memory safety Initial inputs Examples Fuzzer Modified inputs Memory safety check Application Crash? valgrind ◆ Reproduce a real memory bug in a JAR file parser. ◆ Mandatory part: valgrind. – Explain bug: Origin of memory corruption. – Fix bug: Prevent memory corruption. ◆ Finding the bug: radamsa. – Create good seed input for fuzzer. – Experiment with it: What works well? Cyrille Artho, 2018-05-14 10 Optional part: memory leak ◆ The program given in the exercise has (real) memory leaks. ◆ Use valgrind to look at the source of the leaks. ◆ Choose one leak and fix it. Cyrille Artho, 2018-05-14 11 Summary: Memory safety 1. Memory corruption: Serious flaw in C/C++ programs. 2. Memory safety checkers can find memory bugs. 3. Fuzzing can provide inputs for better coverage. Cyrille Artho, 2018-05-14 12.
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