lib/gwp_asan/guarded_pool_allocator.cpp

//===-- guarded_pool_allocator.cpp ------------------------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "gwp_asan/guarded_pool_allocator.h"

#include "gwp_asan/options.h"
#include "gwp_asan/utilities.h"

#include <assert.h>
#include <stddef.h>

using AllocationMetadata = gwp_asan::AllocationMetadata;
using Error = gwp_asan::Error;

namespace gwp_asan {
namespace {
// Forward declare the pointer to the singleton version of this class.
// Instantiated during initialisation, this allows the signal handler
// to find this class in order to deduce the root cause of failures. Must not be
// referenced by users outside this translation unit, in order to avoid
// init-order-fiasco.
GuardedPoolAllocator *SingletonPtr = nullptr;

size_t roundUpTo(size_t Size, size_t Boundary) {
  return (Size + Boundary - 1) & ~(Boundary - 1);
}

uintptr_t getPageAddr(uintptr_t Ptr, uintptr_t PageSize) {
  return Ptr & ~(PageSize - 1);
}

bool isPowerOfTwo(uintptr_t X) { return (X & (X - 1)) == 0; }
} // anonymous namespace

// Gets the singleton implementation of this class. Thread-compatible until
// init() is called, thread-safe afterwards.
GuardedPoolAllocator *GuardedPoolAllocator::getSingleton() {
  return SingletonPtr;
}

void GuardedPoolAllocator::init(const options::Options &Opts) {
  // Note: We return from the constructor here if GWP-ASan is not available.
  // This will stop heap-allocation of class members, as well as mmap() of the
  // guarded slots.
  if (!Opts.Enabled || Opts.SampleRate == 0 ||
      Opts.MaxSimultaneousAllocations == 0)
    return;

  Check(Opts.SampleRate >= 0, "GWP-ASan Error: SampleRate is < 0.");
  Check(Opts.SampleRate < (1 << 30), "GWP-ASan Error: SampleRate is >= 2^30.");
  Check(Opts.MaxSimultaneousAllocations >= 0,
        "GWP-ASan Error: MaxSimultaneousAllocations is < 0.");

  SingletonPtr = this;
  Backtrace = Opts.Backtrace;

  State.VersionMagic = {{AllocatorVersionMagic::kAllocatorVersionMagic[0],
                         AllocatorVersionMagic::kAllocatorVersionMagic[1],
                         AllocatorVersionMagic::kAllocatorVersionMagic[2],
                         AllocatorVersionMagic::kAllocatorVersionMagic[3]},
                        AllocatorVersionMagic::kAllocatorVersion,
                        0};

  State.MaxSimultaneousAllocations = Opts.MaxSimultaneousAllocations;

  const size_t PageSize = getPlatformPageSize();
  // getPageAddr() and roundUpTo() assume the page size to be a power of 2.
  assert((PageSize & (PageSize - 1)) == 0);
  State.PageSize = PageSize;

  size_t PoolBytesRequired =
      PageSize * (1 + State.MaxSimultaneousAllocations) +
      State.MaxSimultaneousAllocations * State.maximumAllocationSize();
  assert(PoolBytesRequired % PageSize == 0);
  void *GuardedPoolMemory = reserveGuardedPool(PoolBytesRequired);

  size_t BytesRequired =
      roundUpTo(State.MaxSimultaneousAllocations * sizeof(*Metadata), PageSize);
  Metadata = reinterpret_cast<AllocationMetadata *>(
      map(BytesRequired, kGwpAsanMetadataName));

  // Allocate memory and set up the free pages queue.
  BytesRequired = roundUpTo(
      State.MaxSimultaneousAllocations * sizeof(*FreeSlots), PageSize);
  FreeSlots =
      reinterpret_cast<size_t *>(map(BytesRequired, kGwpAsanFreeSlotsName));

  // Multiply the sample rate by 2 to give a good, fast approximation for (1 /
  // SampleRate) chance of sampling.
  if (Opts.SampleRate != 1)
    AdjustedSampleRatePlusOne = static_cast<uint32_t>(Opts.SampleRate) * 2 + 1;
  else
    AdjustedSampleRatePlusOne = 2;

  initPRNG();
  getThreadLocals()->NextSampleCounter =
      ((getRandomUnsigned32() % (AdjustedSampleRatePlusOne - 1)) + 1) &
      ThreadLocalPackedVariables::NextSampleCounterMask;

  State.GuardedPagePool = reinterpret_cast<uintptr_t>(GuardedPoolMemory);
  State.GuardedPagePoolEnd =
      reinterpret_cast<uintptr_t>(GuardedPoolMemory) + PoolBytesRequired;

  if (Opts.InstallForkHandlers)
    installAtFork();
}

void GuardedPoolAllocator::disable() {
  PoolMutex.lock();
  BacktraceMutex.lock();
}

void GuardedPoolAllocator::enable() {
  PoolMutex.unlock();
  BacktraceMutex.unlock();
}

void GuardedPoolAllocator::iterate(void *Base, size_t Size, iterate_callback Cb,
                                   void *Arg) {
  uintptr_t Start = reinterpret_cast<uintptr_t>(Base);
  for (size_t i = 0; i < State.MaxSimultaneousAllocations; ++i) {
    const AllocationMetadata &Meta = Metadata[i];
    if (Meta.Addr && !Meta.IsDeallocated && Meta.Addr >= Start &&
        Meta.Addr < Start + Size)
      Cb(Meta.Addr, Meta.RequestedSize, Arg);
  }
}

void GuardedPoolAllocator::uninitTestOnly() {
  if (State.GuardedPagePool) {
    unreserveGuardedPool();
    State.GuardedPagePool = 0;
    State.GuardedPagePoolEnd = 0;
  }
  if (Metadata) {
    unmap(Metadata,
          roundUpTo(State.MaxSimultaneousAllocations * sizeof(*Metadata),
                    State.PageSize));
    Metadata = nullptr;
  }
  if (FreeSlots) {
    unmap(FreeSlots,
          roundUpTo(State.MaxSimultaneousAllocations * sizeof(*FreeSlots),
                    State.PageSize));
    FreeSlots = nullptr;
  }
  *getThreadLocals() = ThreadLocalPackedVariables();
}

// Note, minimum backing allocation size in GWP-ASan is always one page, and
// each slot could potentially be multiple pages (but always in
// page-increments). Thus, for anything that requires less than page size
// alignment, we don't need to allocate extra padding to ensure the alignment
// can be met.
size_t GuardedPoolAllocator::getRequiredBackingSize(size_t Size,
                                                    size_t Alignment,
                                                    size_t PageSize) {
  assert(isPowerOfTwo(Alignment) && "Alignment must be a power of two!");
  assert(Alignment != 0 && "Alignment should be non-zero");
  assert(Size != 0 && "Size should be non-zero");

  if (Alignment <= PageSize)
    return Size;

  return Size + Alignment - PageSize;
}

uintptr_t GuardedPoolAllocator::alignUp(uintptr_t Ptr, size_t Alignment) {
  assert(isPowerOfTwo(Alignment) && "Alignment must be a power of two!");
  assert(Alignment != 0 && "Alignment should be non-zero");
  if ((Ptr & (Alignment - 1)) == 0)
    return Ptr;

  Ptr += Alignment - (Ptr & (Alignment - 1));
  return Ptr;
}

uintptr_t GuardedPoolAllocator::alignDown(uintptr_t Ptr, size_t Alignment) {
  assert(isPowerOfTwo(Alignment) && "Alignment must be a power of two!");
  assert(Alignment != 0 && "Alignment should be non-zero");
  if ((Ptr & (Alignment - 1)) == 0)
    return Ptr;

  Ptr -= Ptr & (Alignment - 1);
  return Ptr;
}

void *GuardedPoolAllocator::allocate(size_t Size, size_t Alignment) {
  // GuardedPagePoolEnd == 0 when GWP-ASan is disabled. If we are disabled, fall
  // back to the supporting allocator.
  if (State.GuardedPagePoolEnd == 0) {
    getThreadLocals()->NextSampleCounter =
        (AdjustedSampleRatePlusOne - 1) &
        ThreadLocalPackedVariables::NextSampleCounterMask;
    return nullptr;
  }

  if (Size == 0)
    Size = 1;
  if (Alignment == 0)
    Alignment = alignof(max_align_t);

  if (!isPowerOfTwo(Alignment) || Alignment > State.maximumAllocationSize() ||
      Size > State.maximumAllocationSize())
    return nullptr;

  size_t BackingSize = getRequiredBackingSize(Size, Alignment, State.PageSize);
  if (BackingSize > State.maximumAllocationSize())
    return nullptr;

  // Protect against recursivity.
  if (getThreadLocals()->RecursiveGuard)
    return nullptr;
  ScopedRecursiveGuard SRG;

  size_t Index;
  {
    ScopedLock L(PoolMutex);
    Index = reserveSlot();
  }

  if (Index == kInvalidSlotID)
    return nullptr;

  uintptr_t SlotStart = State.slotToAddr(Index);
  AllocationMetadata *Meta = addrToMetadata(SlotStart);
  uintptr_t SlotEnd = State.slotToAddr(Index) + State.maximumAllocationSize();
  uintptr_t UserPtr;
  // Randomly choose whether to left-align or right-align the allocation, and
  // then apply the necessary adjustments to get an aligned pointer.
  if (getRandomUnsigned32() % 2 == 0)
    UserPtr = alignUp(SlotStart, Alignment);
  else
    UserPtr = alignDown(SlotEnd - Size, Alignment);

  assert(UserPtr >= SlotStart);
  assert(UserPtr + Size <= SlotEnd);

  // If a slot is multiple pages in size, and the allocation takes up a single
  // page, we can improve overflow detection by leaving the unused pages as
  // unmapped.
  const size_t PageSize = State.PageSize;
  allocateInGuardedPool(
      reinterpret_cast<void *>(getPageAddr(UserPtr, PageSize)),
      roundUpTo(Size, PageSize));

  Meta->RecordAllocation(UserPtr, Size);
  {
    ScopedLock UL(BacktraceMutex);
    Meta->AllocationTrace.RecordBacktrace(Backtrace);
  }

  return reinterpret_cast<void *>(UserPtr);
}

void GuardedPoolAllocator::trapOnAddress(uintptr_t Address, Error E) {
  State.FailureType = E;
  State.FailureAddress = Address;

  // Raise a SEGV by touching first guard page.
  volatile char *p = reinterpret_cast<char *>(State.GuardedPagePool);
  *p = 0;
  // Normally, would be __builtin_unreachable(), but because of
  // https://bugs.llvm.org/show_bug.cgi?id=47480, unreachable will DCE the
  // volatile store above, even though it has side effects.
  __builtin_trap();
}

void GuardedPoolAllocator::stop() {
  getThreadLocals()->RecursiveGuard = true;
  PoolMutex.tryLock();
}

void GuardedPoolAllocator::deallocate(void *Ptr) {
  assert(pointerIsMine(Ptr) && "Pointer is not mine!");
  uintptr_t UPtr = reinterpret_cast<uintptr_t>(Ptr);
  size_t Slot = State.getNearestSlot(UPtr);
  uintptr_t SlotStart = State.slotToAddr(Slot);
  AllocationMetadata *Meta = addrToMetadata(UPtr);
  if (Meta->Addr != UPtr) {
    // If multiple errors occur at the same time, use the first one.
    ScopedLock L(PoolMutex);
    trapOnAddress(UPtr, Error::INVALID_FREE);
  }

  // Intentionally scope the mutex here, so that other threads can access the
  // pool during the expensive markInaccessible() call.
  {
    ScopedLock L(PoolMutex);
    if (Meta->IsDeallocated) {
      trapOnAddress(UPtr, Error::DOUBLE_FREE);
    }

    // Ensure that the deallocation is recorded before marking the page as
    // inaccessible. Otherwise, a racy use-after-free will have inconsistent
    // metadata.
    Meta->RecordDeallocation();

    // Ensure that the unwinder is not called if the recursive flag is set,
    // otherwise non-reentrant unwinders may deadlock.
    if (!getThreadLocals()->RecursiveGuard) {
      ScopedRecursiveGuard SRG;
      ScopedLock UL(BacktraceMutex);
      Meta->DeallocationTrace.RecordBacktrace(Backtrace);
    }
  }

  deallocateInGuardedPool(reinterpret_cast<void *>(SlotStart),
                          State.maximumAllocationSize());

  // And finally, lock again to release the slot back into the pool.
  ScopedLock L(PoolMutex);
  freeSlot(Slot);
}

size_t GuardedPoolAllocator::getSize(const void *Ptr) {
  assert(pointerIsMine(Ptr));
  ScopedLock L(PoolMutex);
  AllocationMetadata *Meta = addrToMetadata(reinterpret_cast<uintptr_t>(Ptr));
  assert(Meta->Addr == reinterpret_cast<uintptr_t>(Ptr));
  return Meta->RequestedSize;
}

AllocationMetadata *GuardedPoolAllocator::addrToMetadata(uintptr_t Ptr) const {
  return &Metadata[State.getNearestSlot(Ptr)];
}

size_t GuardedPoolAllocator::reserveSlot() {
  // Avoid potential reuse of a slot before we have made at least a single
  // allocation in each slot. Helps with our use-after-free detection.
  if (NumSampledAllocations < State.MaxSimultaneousAllocations)
    return NumSampledAllocations++;

  if (FreeSlotsLength == 0)
    return kInvalidSlotID;

  size_t ReservedIndex = getRandomUnsigned32() % FreeSlotsLength;
  size_t SlotIndex = FreeSlots[ReservedIndex];
  FreeSlots[ReservedIndex] = FreeSlots[--FreeSlotsLength];
  return SlotIndex;
}

void GuardedPoolAllocator::freeSlot(size_t SlotIndex) {
  assert(FreeSlotsLength < State.MaxSimultaneousAllocations);
  FreeSlots[FreeSlotsLength++] = SlotIndex;
}

uint32_t GuardedPoolAllocator::getRandomUnsigned32() {
  uint32_t RandomState = getThreadLocals()->RandomState;
  RandomState ^= RandomState << 13;
  RandomState ^= RandomState >> 17;
  RandomState ^= RandomState << 5;
  getThreadLocals()->RandomState = RandomState;
  return RandomState;
}
} // namespace gwp_asan