Fuzzing improvements

fuzz/.gitignore

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+build
+corpus

fuzz/README.md

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+# OpenVPN Fuzzing
+
+## Setup
+The fuzzing setup is handled by Nix inside a `nix-shell` and works both on
+Linux and macOS. Nix is the only dependency (https://nixos.org/download.html).
+
+## Usage
+
+```sh
+$ nix-shell fuzz/shell.nix
+$ autoreconf -i -v -f
+$ ./configure --disable-lz4
+$ cd fuzz
+$ ./openvpn-fuzz.py fuzz base64
+$ ./openvpn-fuzz.py fuzz parse_argv -- -fork=4 -ignore_crashes=1
+$ ./openvpn-fuzz.py coverage base64 parse_argv # specified targets
+$ ./openvpn-fuzz.py coverage # all targets
+$ ./openvpn-fuzz.py coverage --clean # do make clean before and after, use if previously built for fuzzing
+```

fuzz/openvpn-fuzz.py

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+#!/usr/bin/env python
+
+import argparse
+import os
+import platform
+import subprocess
+import sys
+
+TARGETS = [
+           'base64',
+           'buffer',
+           'dhcp',
+           'forward',
+           'list',
+           'misc',
+           'mroute',
+           'mss',
+           'packet_id',
+           'parse_argv',
+           'proxy',
+           'route',
+           'verify_cert',
+          ]
+
+BASE_DIR = os.path.dirname(os.path.realpath(__file__))
+
+def fuzz_target(target, args=[]):
+    build_targets([target])
+    os.makedirs(corpus_dir(target), exist_ok=True)
+    os.chdir(target_dir(target, "fuzzer"))
+    os.execv(target_bin_path(target, "fuzzer"),
+             [target_bin_path(target, "fuzzer"), corpus_dir(target)] + args)
+
+def generate_coverage_report(targets=TARGETS):
+    """
+    If OpenVPN was previously built for fuzzing run `make -C ../ clean` before and after generating coverage.
+    """
+    wd = os.getcwd()
+    build_targets(targets, for_coverage=True)
+    profraws = []
+    object_args = []
+    for target in targets:
+        os.chdir(target_dir(target, "coverage"))
+        profraws.append(target_dir(target, "coverage", "default.profraw"))
+        object_args.append("-object")
+        object_args.append(target_bin_path(target, "coverage"))
+        subprocess.run([target_bin_path(target, "coverage"), corpus_dir(target), "-runs=0"])
+
+    profdata = build_dir("coverage", "combined.profdata")
+    subprocess.run(["llvm-profdata", "merge", "-o", profdata, "-sparse"] + profraws)
+    subprocess.run(["llvm-cov", "show", "--format", "html", f"-instr-profile={profdata}",
+                    "-output-dir", build_dir("coverage", "report")] + object_args)
+    os.chdir(wd)
+
+def triage_parse_argv_crashes():
+    """
+    Filters out false positives that are caused by calling exit.
+    """
+    import pwn
+    target = "parse_argv"
+    for filename in os.listdir(target_dir(target, "fuzzer")):
+        if "crash-" in filename:
+            print("Triaging", filename)
+            with open(target_dir(target, "fuzzer", filename), "rb") as f:
+                argv_raw = f.read()
+                p = pwn.process(executable="../src/openvpn/openvpn", argv=argv_raw.split(b'\x00'))
+                out = p.readall()
+                if b"SIGSEGV" in out or b"smashing" or b"AddressSanitizer" in out:
+                    print(pwn.hexdump(argv_raw))
+                    print(out)
+                    exit(1)
+                p.close()
+
+def build_openvpn(cflags):
+    """
+    Build OpenVPN as usual, assumes `autoconf -f -v -f` and `./configure --disable-lz4` already run.
+    """
+    subprocess.run(["make", "-j", "-C", "../", f"CFLAGS={cflags}"])
+
+def build_targets(targets, for_coverage=False):
+    fuzzer_flags = '-g -fsanitize=address,fuzzer-no-link'
+    coverage_flags = '-g -fprofile-instr-generate -fcoverage-mapping'
+
+    build_subdir = 'coverage' if for_coverage else 'fuzzer'
+    os.makedirs(build_dir(build_subdir), exist_ok=True)
+
+    cflags = coverage_flags if for_coverage else fuzzer_flags
+    cflags += " -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
+    build_openvpn(cflags)
+
+    o_files = []
+    for file in os.listdir("../src/openvpn"):
+        if file.endswith(".o") and file != 'openvpn.o':
+            o_files.append("../src/openvpn/" + file)
+    subprocess.run(["ar", "r", build_dir(build_subdir, "libopenvpn.a")] + o_files)
+
+    subprocess.run(["clang++", "-c", "src/fuzz_randomizer.cpp",
+                    "-o", build_dir(build_subdir, "fuzz_randomizer.o")] +
+                   cflags.split(' '))
+
+    extra_libs = ["-lc++abi", "-lresolv"] if platform.system() == 'Darwin' else ['-lcap-ng']
+
+    for target in targets:
+        os.makedirs(target_dir(target, build_subdir), exist_ok=True)
+        subprocess.run(["clang", "-I../src/openvpn", "-I..", "-I../src/compat", "-I../include",
+                        "-lssl", "-lcrypto", "-llzo2", f"src/fuzz_{target}.c",
+                        build_dir(build_subdir, "libopenvpn.a"),
+                        build_dir(build_subdir, "fuzz_randomizer.o"),
+                        "-o", target_bin_path(target, build_subdir),
+                        "-g", "-fsanitize=address,fuzzer"] +
+                       (coverage_flags.split(' ') if for_coverage else []) +
+                       extra_libs)
+
+def build_dir(subdir, file=''):
+    """
+    There are two build flavors that live in their own subdirs: coverage and fuzzer.
+    """
+    return os.path.join(BASE_DIR, "build", subdir, file)
+
+def target_dir(target, subdir, file=''):
+    return os.path.join(build_dir(subdir), f"fuzz_{target}", file)
+
+def corpus_dir(target):
+    return os.path.join(BASE_DIR, "corpus", f"fuzz_{target}")
+
+def target_bin(target):
+    return f"fuzz_{target}"
+
+def target_bin_path(target, subdir):
+    return target_dir(target, subdir, target_bin(target))
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    subparsers = parser.add_subparsers(dest="subcommand")
+    fuzz_parser = subparsers.add_parser('fuzz')
+    fuzz_parser.add_argument('target', type=str)
+    fuzz_parser.add_argument('libfuzzer_args', type=str, nargs='*')
+    coverage_parser = subparsers.add_parser('coverage')
+    coverage_parser.add_argument('targets', type=str, nargs='*')
+    coverage_parser.add_argument('--clean', action=argparse.BooleanOptionalAction)
+
+    args = parser.parse_args()
+    if args.subcommand == 'fuzz':
+        # ./openvpn-fuzz.py fuzz proxy -- -fork=4 -ignore_crashes=1
+        fuzz_target(args.target, args.libfuzzer_args)
+    elif args.subcommand == 'coverage':
+        if args.clean:
+            subprocess.run(["make", "-C", "../", "clean"])
+
+        if args.targets:
+            generate_coverage_report(args.targets)
+        else:
+            generate_coverage_report()
+
+        if args.clean:
+            subprocess.run(["make", "-C", "../", "clean"])
+    else:
+        parser.print_help()

fuzz/shell.nix

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+with import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/e58a7747db96c23b8a977e7c1bbfc5753b81b6fa.tar.gz") {};
+
+let llvmPackages = llvmPackages_14;
+in llvmPackages.stdenv.mkDerivation {
+  name = "openvpn-fuzz";
+  buildInputs = [
+    autoconf
+    automake
+    m4
+    libtool
+    pkg-config
+    openssl_1_1
+    lz4
+    lzo
+    pam
+    llvmPackages.llvm
+    python3Packages.pwntools
+  ] ++ lib.optional (!stdenv.isDarwin) libcap_ng;
+}

fuzz/src/fuzz_base64.c

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+/* Copyright 2021 Google LLC
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+      http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "base64.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size > 500) {
+    return 0;
+  }
+
+  char *new_str = (char *)malloc(size + 1);
+  if (new_str == NULL) {
+    return 0;
+  }
+  memcpy(new_str, data, size);
+  new_str[size] = '\0';
+
+  char *str = NULL;
+  openvpn_base64_encode(data, size, &str);
+  if(str != NULL) {
+    free(str);
+  }
+
+  uint16_t outsize = 10000;
+  char *output_buf = (char *)malloc(outsize);
+  openvpn_base64_decode(new_str, output_buf, outsize);
+  free(output_buf);
+
+  free(new_str);
+  return 0;
+}