[libFuzzer] Unpoison parameters before calling user callback.

[morehouse]

Summary

Fixes an MSan false positive when compiling with
-fsanitize=memory,fuzzer.

See https://github.com/google/oss-fuzz/issues/2369 for more details.

[kcc]

LGTM, thanks!

lib/fuzzer/FuzzerExtFunctions.def

 EXT_FUNC(__sanitizer_get_module_and_offset_for_pc, int,
          (void *pc, char *module_path,
          size_t module_path_len,void **pc_offset), false);
 EXT_FUNC(__sanitizer_set_death_callback, void, (void (*)(void)), true);
 EXT_FUNC(__sanitizer_set_report_fd, void, (void*), false);
 EXT_FUNC(__msan_scoped_disable_interceptor_checks, void, (), false);
 EXT_FUNC(__msan_scoped_enable_interceptor_checks, void, (), false);
 EXT_FUNC(__msan_unpoison, void, (const volatile void *, size_t size), false);
+EXT_FUNC(__msan_unpoison_param, void, (size_t n), false);

lib/fuzzer/FuzzerLoop.cpp

void Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {
   TotalNumberOfRuns++;
   assert(InFuzzingThread());
   // We copy the contents of Unit into a separate heap buffer
   // so that we reliably find buffer overflows in it.
   uint8_t *DataCopy = new uint8_t[Size];
   memcpy(DataCopy, Data, Size);
   if (EF->__msan_unpoison)
     EF->__msan_unpoison(DataCopy, Size);
+  if (EF->__msan_unpoison_param)
+    EF->__msan_unpoison_param(2);
   if (CurrentUnitData && CurrentUnitData != Data)
     memcpy(CurrentUnitData, Data, Size);
   CurrentUnitSize = Size;
   {
     ScopedEnableMsanInterceptorChecks S;
     AllocTracer.Start(Options.TraceMalloc);
     UnitStartTime = system_clock::now();
     TPC.ResetMaps();
bool NewCov = RunOne(CurrentUnitData, Size, /*MayDeleteFile=*/true, &II,
                          &FoundUniqFeatures);
     TryDetectingAMemoryLeak(CurrentUnitData, Size,
                             /*DuringInitialCorpusExecution*/ false);
     if (NewCov) {
       ReportNewCoverage(&II, {CurrentUnitData, CurrentUnitData + Size});
       break;  // We will mutate this input more in the next rounds.
     }
     if (Options.ReduceDepth && !FoundUniqFeatures)
-        break;
   }
 }
 
 void Fuzzer::PurgeAllocator() {
   if (Options.PurgeAllocatorIntervalSec < 0 || !EF->__sanitizer_purge_allocator)
     return;
   if (duration_cast<seconds>(system_clock::now() -
                              LastAllocatorPurgeAttemptTime)

test/fuzzer/MsanParamUnpoison.cpp

+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Triggers the bug described here:
+// https://github.com/google/oss-fuzz/issues/2369#issuecomment-490240627
+//
+// In a nutshell, MSan's parameter shadow does not get unpoisoned before calls
+// to LLVMFuzzerTestOneInput.  This test case causes the parameter shadow to be
+// poisoned by the call to foo(), which will trigger an MSan false positive on
+// the Size == 0 check if the parameter shadow is still poisoned.
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
+volatile int zero = 0;
+__attribute__((noinline)) int foo(int arg1, int arg2) { return zero; }
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  if (Size == 0)
+    return 0;
+
+  // Pass uninitialized values to foo().  Since foo doesn't do anything with
+  // them, MSan should not report an error here.
+  int a, b;
+  return foo(a, b);
+}

test/fuzzer/msan-param-unpoison.test

+REQUIRES: msan
+RUN: %msan_compiler %S/MsanParamUnpoison.cpp -o %t
+RUN: %run %t -seed=1 -runs=1000 2>&1 | FileCheck %s
+
+CHECK-NOT: MemorySanitizer: use-of-uninitialized-value