gecko-dev/security/sandbox/linux/SandboxChroot.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "SandboxChroot.h"

#include "SandboxLogging.h"
#include "LinuxCapabilities.h"

#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "base/posix/eintr_wrapper.h"
#include "mozilla/Assertions.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/NullPtr.h"

#define MOZ_ALWAYS_ZERO(e) MOZ_ALWAYS_TRUE((e) == 0)

namespace mozilla {

SandboxChroot::SandboxChroot()
{
  pthread_mutexattr_t attr;
  MOZ_ALWAYS_ZERO(pthread_mutexattr_init(&attr));
  MOZ_ALWAYS_ZERO(pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK));
  MOZ_ALWAYS_ZERO(pthread_mutex_init(&mMutex, &attr));
  MOZ_ALWAYS_ZERO(pthread_cond_init(&mWakeup, nullptr));
  mCommand = NO_THREAD;
}

SandboxChroot::~SandboxChroot()
{
  SendCommand(JUST_EXIT);
  MOZ_ALWAYS_ZERO(pthread_mutex_destroy(&mMutex));
  MOZ_ALWAYS_ZERO(pthread_cond_destroy(&mWakeup));
}

bool
SandboxChroot::SendCommand(Command aComm)
{
  MOZ_ALWAYS_ZERO(pthread_mutex_lock(&mMutex));
  if (mCommand == NO_THREAD) {
    MOZ_RELEASE_ASSERT(aComm == JUST_EXIT);
    MOZ_ALWAYS_ZERO(pthread_mutex_unlock(&mMutex));
    return false;
  } else {
    MOZ_ASSERT(mCommand == NO_COMMAND);
    mCommand = aComm;
    MOZ_ALWAYS_ZERO(pthread_mutex_unlock(&mMutex));
    MOZ_ALWAYS_ZERO(pthread_cond_signal(&mWakeup));
    void *retval;
    if (pthread_join(mThread, &retval) != 0 || retval != nullptr) {
      MOZ_CRASH("Failed to stop privileged chroot thread");
    }
    MOZ_ASSERT(mCommand == NO_THREAD);
  }
  return true;
}

static void
AlwaysClose(int fd)
{
  if (IGNORE_EINTR(close(fd)) != 0) {
    SANDBOX_LOG_ERROR("close: %s", strerror(errno));
    MOZ_CRASH("failed to close()");
  }
}

static int
OpenDeletedDirectory()
{
  // We don't need this directory to persist between invocations of
  // the program (nor need it to be cleaned up if something goes wrong
  // here, because mkdtemp will choose a fresh name), so /tmp as
  // specified by FHS is adequate.
  //
  // However, this needs a filesystem where a deleted directory can
  // still be used, and /tmp is sometimes not that; e.g., aufs(5),
  // often used for containers, will cause the chroot() to fail with
  // ESTALE (bug 1162965).  So this uses /dev/shm if possible instead.
  char tmpPath[] = "/tmp/mozsandbox.XXXXXX";
  char shmPath[] = "/dev/shm/mozsandbox.XXXXXX";
  char* path;
  if (mkdtemp(shmPath)) {
    path = shmPath;
  } else if (mkdtemp(tmpPath)) {
    path = tmpPath;
  } else {
    SANDBOX_LOG_ERROR("mkdtemp: %s", strerror(errno));
    return -1;
  }
  int fd = HANDLE_EINTR(open(path, O_RDONLY | O_DIRECTORY));
  if (fd < 0) {
    SANDBOX_LOG_ERROR("open %s: %s", path, strerror(errno));
    // Try to clean up.  Shouldn't fail, but livable if it does.
    DebugOnly<bool> ok = HANDLE_EINTR(rmdir(path)) == 0;
    MOZ_ASSERT(ok);
    return -1;
  }
  if (HANDLE_EINTR(rmdir(path)) != 0) {
    SANDBOX_LOG_ERROR("rmdir %s: %s", path, strerror(errno));
    AlwaysClose(fd);
    return -1;
  }
  return fd;
}

bool
SandboxChroot::Prepare()
{
  LinuxCapabilities caps;
  if (!caps.GetCurrent() || !caps.Effective(CAP_SYS_CHROOT)) {
    SANDBOX_LOG_ERROR("don't have permission to chroot");
    return false;
  }
  mFd = OpenDeletedDirectory();
  if (mFd < 0) {
    SANDBOX_LOG_ERROR("failed to create empty directory for chroot");
    return false;
  }
  MOZ_ALWAYS_ZERO(pthread_mutex_lock(&mMutex));
  MOZ_ASSERT(mCommand == NO_THREAD);
  if (pthread_create(&mThread, nullptr, StaticThreadMain, this) != 0) {
    MOZ_ALWAYS_ZERO(pthread_mutex_unlock(&mMutex));
    SANDBOX_LOG_ERROR("pthread_create: %s", strerror(errno));
    return false;
  }
  while (mCommand != NO_COMMAND) {
    MOZ_ASSERT(mCommand == NO_THREAD);
    MOZ_ALWAYS_ZERO(pthread_cond_wait(&mWakeup, &mMutex));
  }
  MOZ_ALWAYS_ZERO(pthread_mutex_unlock(&mMutex));
  return true;
}

void
SandboxChroot::Invoke()
{
  MOZ_ALWAYS_TRUE(SendCommand(DO_CHROOT));
}

static bool
ChrootToFileDesc(int fd)
{
  if (fchdir(fd) != 0) {
    SANDBOX_LOG_ERROR("fchdir: %s", strerror(errno));
    return false;
  }
  if (chroot(".") != 0) {
    SANDBOX_LOG_ERROR("chroot: %s", strerror(errno));
    return false;
  }
  return true;
}

/* static */ void*
SandboxChroot::StaticThreadMain(void* aVoidPtr)
{
  static_cast<SandboxChroot*>(aVoidPtr)->ThreadMain();
  return nullptr;
}

void
SandboxChroot::ThreadMain()
{
  // First, drop everything that isn't CAP_SYS_CHROOT.  (This code
  // assumes that this thread already has effective CAP_SYS_CHROOT,
  // because Prepare() checked for it before creating this thread.)
  LinuxCapabilities caps;
  caps.Effective(CAP_SYS_CHROOT) = true;
  if (!caps.SetCurrent()) {
    SANDBOX_LOG_ERROR("capset: %s", strerror(errno));
    MOZ_CRASH("Can't limit chroot thread's capabilities");
  }

  MOZ_ALWAYS_ZERO(pthread_mutex_lock(&mMutex));
  MOZ_ASSERT(mCommand == NO_THREAD);
  mCommand = NO_COMMAND;
  MOZ_ALWAYS_ZERO(pthread_cond_signal(&mWakeup));
  while (mCommand == NO_COMMAND) {
    MOZ_ALWAYS_ZERO(pthread_cond_wait(&mWakeup, &mMutex));
  }
  if (mCommand == DO_CHROOT) {
    MOZ_ASSERT(mFd >= 0);
    if (!ChrootToFileDesc(mFd)) {
      MOZ_CRASH("Failed to chroot");
    }
  } else {
    MOZ_ASSERT(mCommand == JUST_EXIT);
  }
  if (mFd >= 0) {
    AlwaysClose(mFd);
    mFd = -1;
  }
  mCommand = NO_THREAD;
  MOZ_ALWAYS_ZERO(pthread_mutex_unlock(&mMutex));
  // Drop the remaining capabilities; see note in SandboxChroot.h
  // about the potential unreliability of pthread_join.
  if (!LinuxCapabilities().SetCurrent()) {
    MOZ_CRASH("can't drop capabilities");
  }
}

} // namespace mozilla

#undef MOZ_ALWAYS_ZERO