For EIP-4844, Ethereum purchasers want the power to compute and confirm KZG commitments. Reasonably than every shopper rolling their very own crypto, researchers and builders got here collectively to put in writing c-kzg-4844, a comparatively small C library with bindings for higher-level languages. The concept was to create a strong and environment friendly cryptographic library that each one purchasers may use. The Protocol Safety Analysis group on the Ethereum Basis had the chance to evaluation and enhance this library. This weblog put up will talk about some issues we do to make C tasks safer.
Fuzz
Fuzzing is a dynamic code testing approach that includes offering random inputs to find bugs in a program. LibFuzzer and afl++ are two well-liked fuzzing frameworks for C tasks. They’re each in-process, coverage-guided, evolutionary fuzzing engines. For c-kzg-4844, we used LibFuzzer since we had been already well-integrated with LLVM venture’s different choices.
Here is the fuzzer for verify_kzg_proof, one among c-kzg-4844’s features:
#embody "../base_fuzz.h" static const size_t COMMITMENT_OFFSET = 0; static const size_t Z_OFFSET = COMMITMENT_OFFSET + BYTES_PER_COMMITMENT; static const size_t Y_OFFSET = Z_OFFSET + BYTES_PER_FIELD_ELEMENT; static const size_t PROOF_OFFSET = Y_OFFSET + BYTES_PER_FIELD_ELEMENT; static const size_t INPUT_SIZE = PROOF_OFFSET + BYTES_PER_PROOF; int LLVMFuzzerTestOneInput(const uint8_t* information, size_t dimension) { initialize(); if (dimension == INPUT_SIZE) { bool okay; verify_kzg_proof( &okay, (const Bytes48 *)(information + COMMITMENT_OFFSET), (const Bytes32 *)(information + Z_OFFSET), (const Bytes32 *)(information + Y_OFFSET), (const Bytes48 *)(information + PROOF_OFFSET), &s ); } return 0; }
When executed, that is what the output appears to be like like. If there have been an issue, it might write the enter to disk and cease executing. Ideally, you need to be capable to reproduce the issue.
There’s additionally differential fuzzing, which is a method which fuzzes two or extra implementations of the identical interface and compares the outputs. For a given enter, if the output is totally different, and also you anticipated them to be the identical, you understand one thing is mistaken. This system may be very well-liked in Ethereum as a result of we wish to have a number of implementations of the identical factor. This diversification offers an additional stage of security, understanding that if one implementation had been flawed the others could not have the identical problem.
For KZG libraries, we developed kzg-fuzz which differentially fuzzes c-kzg-4844 (by way of its Golang bindings) and go-kzg-4844. To this point, there have not been any variations.
Protection
Subsequent, we used llvm-profdata and llvm-cov to generate a protection report from operating the exams. This can be a nice method to confirm code is executed (“coated”) and examined. See the coverage goal in c-kzg-4844’s Makefile for an instance of how you can generate this report.
When this goal is run (i.e., make protection) it produces a desk that serves as a high-level overview of how a lot of every perform is executed. The exported features are on the prime and the non-exported (static) features are on the underside.
There’s quite a lot of inexperienced within the desk above, however there’s some yellow and purple too. To find out what’s and is not being executed, check with the HTML file (protection.html) that was generated. This webpage exhibits your entire supply file and highlights non-executed code in purple. On this venture’s case, many of the non-executed code offers with hard-to-test error instances equivalent to reminiscence allocation failures. For instance, this is some non-executed code:
Firstly of this perform, it checks that the trusted setup is large enough to carry out a pairing verify. There is not a take a look at case which offers an invalid trusted setup, so this does not get executed. Additionally, as a result of we solely take a look at with the proper trusted setup, the results of is_monomial_form is all the time the identical and does not return the error worth.
Profile
We do not advocate this for all tasks, however since c-kzg-4844 is a efficiency important library we predict it is vital to profile its exported features and measure how lengthy they take to execute. This may also help establish inefficiencies which may doubtlessly DoS nodes. For this, we used gperftools (Google Efficiency Instruments) as an alternative of llvm-xray as a result of we discovered it to be extra feature-rich and simpler to make use of.
The next is a straightforward instance which profiles my_function. Profiling works by checking which instruction is being executed occasionally. If a perform is quick sufficient, it is probably not observed by the profiler. To scale back the prospect of this, you could must name your perform a number of occasions. On this instance, we name my_function 1000 occasions.
#embodyint task_a(int n) { if (n return task_a(n - 1) * n; } int task_b(int n) { if (n return task_b(n - 2) + n; } void my_function(void) { for (int i = 0; i if (i % 2 == 0) { task_a(i); } else { task_b(i); } } } int predominant(void) { ProfilerStart("instance.prof"); for (int i = 0; i my_function(); } ProfilerStop(); return 0; }
Use ProfilerStart(“
Right here is the graph generated from the command above:
Here is an even bigger instance from one among c-kzg-4844’s features. The next picture is the profiling graph for compute_blob_kzg_proof. As you’ll be able to see, 80% of this perform’s time is spent performing Montgomery multiplications. That is anticipated.
Reverse
Subsequent, view your binary in a software program reverse engineering (SRE) device equivalent to Ghidra or IDA. These instruments may also help you perceive how high-level constructs are translated into low-level machine code. We predict it helps to evaluation your code this manner; like how studying a paper in a unique font will drive your mind to interpret sentences otherwise. It is also helpful to see what sort of optimizations your compiler makes. It is uncommon, however generally the compiler will optimize out one thing which it deemed pointless. Maintain a watch out for this, one thing like this really occurred in c-kzg-4844, some of the tests were being optimized out.
While you view a decompiled perform, it won’t have variable names, complicated sorts, or feedback. When compiled, this data is not included within the binary. It will likely be as much as you to reverse engineer this. You may typically see features are inlined right into a single perform, a number of variables declared in code are optimized right into a single buffer, and the order of checks are totally different. These are simply compiler optimizations and are typically wonderful. It could assist to construct your binary with DWARF debugging data; most SREs can analyze this part to offer higher outcomes.
For instance, that is what blob_to_kzg_commitment initially appears to be like like in Ghidra:
With somewhat work, you’ll be able to rename variables and add feedback to make it simpler to learn. Here is what it may appear to be after a couple of minutes:
Static Evaluation
Clang comes built-in with the Clang Static Analyzer, which is a wonderful static evaluation device that may establish many issues that the compiler will miss. Because the identify “static” suggests, it examines code with out executing it. That is slower than the compiler, however lots quicker than “dynamic” evaluation instruments which execute code.
Here is a easy instance which forgets to free arr (and has one other downside however we are going to discuss extra about that later). The compiler won’t establish this, even with all warnings enabled as a result of technically that is utterly legitimate code.
#embodyint predominant(void) { int* arr = malloc(5 * sizeof(int)); arr[5] = 42; return 0; }
The unix.Malloc checker will establish that arr wasn’t freed. The road within the warning message is a bit deceptive, nevertheless it is sensible if you concentrate on it; the analyzer reached the return assertion and observed that the reminiscence hadn’t been freed.
Not all the findings are that straightforward although. Here is a discovering that Clang Static Analyzer present in c-kzg-4844 when initially launched to the venture:
Given an sudden enter, it was potential to shift this worth by 32 bits which is undefined conduct. The answer was to limit the enter with CHECK(log2_pow2(n) != 0) in order that this was unimaginable. Good job, Clang Static Analyzer!
Sanitize
Santizers are dynamic evaluation instruments which instrument (add directions) to applications which might level out points throughout execution. These are significantly helpful at discovering frequent errors related to reminiscence dealing with. Clang comes built-in with a number of sanitizers; listed here are the 4 we discover most helpful and straightforward to make use of.
Deal with
AddressSanitizer (ASan) is a quick reminiscence error detector which might establish out-of-bounds accesses, use-after-free, use-after-return, use-after-scope, double-free, and reminiscence leaks.
Right here is similar instance from earlier. It forgets to free arr and it’ll set the sixth ingredient in a 5 ingredient array. This can be a easy instance of a heap-buffer-overflow:
#embodyint predominant(void) { int* arr = malloc(5 * sizeof(int)); arr[5] = 42; return 0; }
When compiled with -fsanitize=deal with and executed, it would output the next error message. This factors you in route (a 4-byte write in predominant). This binary may very well be seen in a disassembler to determine precisely which instruction (at predominant+0x84) is inflicting the issue.
Equally, this is an instance the place it finds a heap-use-after-free:
#embodyint predominant(void) { int *arr = malloc(5 * sizeof(int)); free(arr); return arr[2]; }
It tells you that there is a 4-byte learn of freed reminiscence at predominant+0x8c.
Reminiscence
MemorySanitizer (MSan) is a detector of uninitialized reads. Here is a easy instance which reads (and returns) an uninitialized worth:
int predominant(void) { int information[2]; return information[0]; }
When compiled with -fsanitize=reminiscence and executed, it would output the next error message:
Undefined Conduct
UndefinedBehaviorSanitizer (UBSan) detects undefined conduct, which refers back to the state of affairs the place a program’s conduct is unpredictable and never specified by the langauge normal. Some frequent examples of this are accessing out-of-bounds reminiscence, dereferencing an invalid pointer, studying uninitialized variables, and overflow of a signed integer. For instance, right here we increment INT_MAX which is undefined conduct.
#embodyint predominant(void) { int a = INT_MAX; return a + 1; }
When compiled with -fsanitize=undefined and executed, it would output the next error message which tells us precisely the place the issue is and what the situations are:
Thread
ThreadSanitizer (TSan) detects information races, which might happen in multi-threaded applications when two or extra threads entry a shared reminiscence location on the identical time. This example introduces unpredictability and might result in undefined conduct. Here is an instance wherein two threads increment a world counter variable. There are not any locks or semaphores, so it is completely potential that these two threads will increment the variable on the identical time.
#embodyint counter = 0; void *increment(void *arg) { (void)arg; for (int i = 0; i counter++; return NULL; } int predominant(void) { pthread_t thread1, thread2; pthread_create(&thread1, NULL, increment, NULL); pthread_create(&thread2, NULL, increment, NULL); pthread_join(thread1, NULL); pthread_join(thread2, NULL); return 0; }
When compiled with -fsanitize=thread and executed, it would output the next error message:
This error message tells us that there is a information race. In two threads, the increment perform is writing to the identical 4 bytes on the identical time. It even tells us that the reminiscence is counter.
Valgrind
Valgrind is a strong instrumentation framework for constructing dynamic evaluation instruments, however its finest identified for figuring out reminiscence errors and leaks with its built-in Memcheck device.
The next picture exhibits the output from operating c-kzg-4844’s exams with Valgrind. Within the purple field is a legitimate discovering for a “conditional soar or transfer [that] relies on uninitialized worth(s).”
This identified an edge case in expand_root_of_unity. If the mistaken root of unity or width had been supplied, it was potential that the loop will break earlier than out[width] was initialized. On this state of affairs, the ultimate verify would depend upon an uninitialized worth.
static C_KZG_RET expand_root_of_unity( fr_t *out, const fr_t *root, uint64_t width ) { out[0] = FR_ONE; out[1] = *root; for (uint64_t i = 2; !fr_is_one(&out[i - 1]); i++) { CHECK(i blst_fr_mul(&out[i], &out[i - 1], root); } CHECK(fr_is_one(&out[width])); return C_KZG_OK; }
Safety Evaluation
After improvement stabilizes, it has been totally examined, and your group has manually reviewed the codebase themselves a number of occasions, it is time to get a safety evaluation by a good safety group. This may not be a stamp of approval, nevertheless it exhibits that your venture is a minimum of considerably safe. Bear in mind there isn’t a such factor as excellent safety. There’ll all the time be the chance of vulnerabilities.
For c-kzg-4844 and go-kzg-4844, the Ethereum Basis contracted Sigma Prime to conduct a safety evaluation. They produced this report with 8 findings. It comprises one important vulnerability in go-kzg-4844 that was a very good discover. The BLS12-381 library that go-kzg-4844 makes use of, gnark-crypto, had a bug which allowed invalid G1 and G2 factors to be sucessfully decoded. Had this not been fastened, this might have resulted in a consensus bug (a disagreement between implementations) in Ethereum.
Bug Bounty
If a vulnerability in your venture may very well be exploited for positive factors, like it’s for Ethereum, think about organising a bug bounty program. This permits safety researchers, or anybody actually, to submit vulnerability experiences in alternate for cash. Usually, that is particularly for findings which might show that an exploit is feasible. If the bug bounty payouts are affordable, bug finders will notify you of the bug slightly than exploiting it or promoting it to a different celebration. We advocate beginning your bug bounty program after the findings from the primary safety evaluation are resolved; ideally, the safety evaluation would price lower than the bug bounty payouts.
Conclusion
The event of sturdy C tasks, particularly within the important area of blockchain and cryptocurrencies, requires a multi-faceted method. Given the inherent vulnerabilities related to the C language, a mixture of finest practices and instruments is important for producing resilient software program. We hope our experiences and findings from our work with c-kzg-4844 present precious insights and finest practices for others embarking on related tasks.
