path: root/chrome/nacl/nacl_helper_linux.cc

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// A mini-zygote specifically for Native Client.

#include "chrome/common/nacl_helper_linux.h"

#include <errno.h>
#include <fcntl.h>
#include <link.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>

#include <string>
#include <vector>

#include "base/at_exit.h"
#include "base/command_line.h"
#include "base/json/string_escape.h"
#include "base/logging.h"
#include "base/message_loop.h"
#include "base/posix/eintr_wrapper.h"
#include "base/posix/global_descriptors.h"
#include "base/posix/unix_domain_socket_linux.h"
#include "base/rand_util.h"
#include "chrome/nacl/nacl_listener.h"
#include "crypto/nss_util.h"
#include "ipc/ipc_descriptors.h"
#include "ipc/ipc_switches.h"
#include "sandbox/linux/services/libc_urandom_override.h"

namespace {

// The child must mimic the behavior of zygote_main_linux.cc on the child
// side of the fork. See zygote_main_linux.cc:HandleForkRequest from
//   if (!child) {
// Note: this code doesn't attempt to support the SECCOMP sandbox.
void BecomeNaClLoader(const std::vector<int>& child_fds,
                      size_t prereserved_sandbox_size) {
  VLOG(1) << "NaCl loader: setting up IPC descriptor";
  // don't need zygote FD any more
  if (HANDLE_EINTR(close(kNaClZygoteDescriptor)) != 0)
    LOG(ERROR) << "close(kNaClZygoteDescriptor) failed.";
  base::GlobalDescriptors::GetInstance()->Set(kPrimaryIPCChannel,
                                              child_fds[kNaClBrowserFDIndex]);

  base::MessageLoopForIO main_message_loop;
  NaClListener listener;
  listener.set_prereserved_sandbox_size(prereserved_sandbox_size);
  listener.Listen();
  _exit(0);
}

// Some of this code was lifted from
// content/browser/zygote_main_linux.cc:ForkWithRealPid()
void HandleForkRequest(const std::vector<int>& child_fds,
                       size_t prereserved_sandbox_size) {
  VLOG(1) << "nacl_helper: forking";
  pid_t childpid = fork();
  if (childpid < 0) {
    perror("fork");
    LOG(ERROR) << "*** HandleForkRequest failed\n";
    // fall through to parent case below
  } else if (childpid == 0) {  // In the child process.
    bool validack = false;
    const size_t kMaxReadSize = 1024;
    char buffer[kMaxReadSize];
    // Wait until the parent process has discovered our PID.  We
    // should not fork any child processes (which the seccomp
    // sandbox does) until then, because that can interfere with the
    // parent's discovery of our PID.
    const int nread = HANDLE_EINTR(read(child_fds[kNaClParentFDIndex], buffer,
                                        kMaxReadSize));
    const std::string switch_prefix = std::string("--") +
        switches::kProcessChannelID + std::string("=");
    const size_t len = switch_prefix.length();

    if (nread < 0) {
      perror("read");
      LOG(ERROR) << "read returned " << nread;
    } else if (nread > static_cast<int>(len)) {
      if (switch_prefix.compare(0, len, buffer, 0, len) == 0) {
        VLOG(1) << "NaCl loader is synchronised with Chrome zygote";
        CommandLine::ForCurrentProcess()->AppendSwitchASCII(
            switches::kProcessChannelID,
            std::string(&buffer[len], nread - len));
        validack = true;
      }
    }
    if (HANDLE_EINTR(close(child_fds[kNaClDummyFDIndex])) != 0)
      LOG(ERROR) << "close(child_fds[kNaClDummyFDIndex]) failed";
    if (HANDLE_EINTR(close(child_fds[kNaClParentFDIndex])) != 0)
      LOG(ERROR) << "close(child_fds[kNaClParentFDIndex]) failed";
    if (validack) {
      BecomeNaClLoader(child_fds, prereserved_sandbox_size);
    } else {
      LOG(ERROR) << "Failed to synch with zygote";
    }
    // NOTREACHED
    return;
  }
  // I am the parent.
  // First, close the dummy_fd so the sandbox won't find me when
  // looking for the child's pid in /proc. Also close other fds.
  for (size_t i = 0; i < child_fds.size(); i++) {
    if (HANDLE_EINTR(close(child_fds[i])) != 0)
      LOG(ERROR) << "close(child_fds[i]) failed";
  }
  VLOG(1) << "nacl_helper: childpid is " << childpid;
  // Now tell childpid to the Chrome zygote.
  if (HANDLE_EINTR(send(kNaClZygoteDescriptor,
                        &childpid, sizeof(childpid), MSG_EOR))
      != sizeof(childpid)) {
    LOG(ERROR) << "*** send() to zygote failed";
  }
}

// This is a poor man's check on whether we are sandboxed.
bool IsSandboxed() {
  int proc_fd = open("/proc/self/exe", O_RDONLY);
  if (proc_fd >= 0) {
    HANDLE_EINTR(close(proc_fd));
    return false;
  }
  return true;
}

}  // namespace

static const char kNaClHelperReservedAtZero[] = "reserved_at_zero";
static const char kNaClHelperRDebug[] = "r_debug";

// Since we were started by nacl_helper_bootstrap rather than in the
// usual way, the debugger cannot figure out where our executable
// or the dynamic linker or the shared libraries are in memory,
// so it won't find any symbols.  But we can fake it out to find us.
//
// The zygote passes --r_debug=0xXXXXXXXXXXXXXXXX.
// nacl_helper_bootstrap replaces the Xs with the address of its _r_debug
// structure.  The debugger will look for that symbol by name to
// discover the addresses of key dynamic linker data structures.
// Since all it knows about is the original main executable, which
// is the bootstrap program, it finds the symbol defined there.  The
// dynamic linker's structure is somewhere else, but it is filled in
// after initialization.  The parts that really matter to the
// debugger never change.  So we just copy the contents of the
// dynamic linker's structure into the address provided by the option.
// Hereafter, if someone attaches a debugger (or examines a core dump),
// the debugger will find all the symbols in the normal way.
static void CheckRDebug(char* argv0) {
  std::string r_debug_switch_value =
      CommandLine::ForCurrentProcess()->GetSwitchValueASCII(kNaClHelperRDebug);
  if (!r_debug_switch_value.empty()) {
    char* endp;
    uintptr_t r_debug_addr = strtoul(r_debug_switch_value.c_str(), &endp, 0);
    if (r_debug_addr != 0 && *endp == '\0') {
      r_debug* bootstrap_r_debug = reinterpret_cast<r_debug*>(r_debug_addr);
      *bootstrap_r_debug = _r_debug;

      // Since the main executable (the bootstrap program) does not
      // have a dynamic section, the debugger will not skip the
      // first element of the link_map list as it usually would for
      // an executable or PIE that was loaded normally.  But the
      // dynamic linker has set l_name for the PIE to "" as is
      // normal for the main executable.  So the debugger doesn't
      // know which file it is.  Fill in the actual file name, which
      // came in as our argv[0].
      link_map* l = _r_debug.r_map;
      if (l->l_name[0] == '\0')
        l->l_name = argv0;
    }
  }
}

// The zygote passes --reserved_at_zero=0xXXXXXXXXXXXXXXXX.
// nacl_helper_bootstrap replaces the Xs with the amount of prereserved
// sandbox memory.
//
// CheckReservedAtZero parses the value of the argument reserved_at_zero
// and returns the amount of prereserved sandbox memory.
static size_t CheckReservedAtZero() {
  size_t prereserved_sandbox_size = 0;
  std::string reserved_at_zero_switch_value =
      CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
          kNaClHelperReservedAtZero);
  if (!reserved_at_zero_switch_value.empty()) {
    char* endp;
    prereserved_sandbox_size =
        strtoul(reserved_at_zero_switch_value.c_str(), &endp, 0);
    if (*endp != '\0')
      LOG(ERROR) << "Could not parse reserved_at_zero argument value of "
                 << reserved_at_zero_switch_value;
  }
  return prereserved_sandbox_size;
}

#if defined(ADDRESS_SANITIZER)
// Do not install the SIGSEGV handler in ASan. This should make the NaCl
// platform qualification test pass.
static const char kAsanDefaultOptionsNaCl[] = "handle_segv=0";

// Override the default ASan options for the NaCl helper.
// __asan_default_options should not be instrumented, because it is called
// before ASan is initialized.
extern "C"
__attribute__((no_address_safety_analysis))
const char* __asan_default_options() {
  return kAsanDefaultOptionsNaCl;
}
#endif

int main(int argc, char* argv[]) {
  CommandLine::Init(argc, argv);
  base::AtExitManager exit_manager;
  base::RandUint64();  // acquire /dev/urandom fd before sandbox is raised
  // Allows NSS to fopen() /dev/urandom.
  sandbox::InitLibcUrandomOverrides();
#if defined(USE_NSS)
  // Configure NSS for use inside the NaCl process.
  // The fork check has not caused problems for NaCl, but this appears to be
  // best practice (see other places LoadNSSLibraries is called.)
  crypto::DisableNSSForkCheck();
  // Without this line on Linux, HMAC::Init will instantiate a singleton that
  // in turn attempts to open a file.  Disabling this behavior avoids a ~70 ms
  // stall the first time HMAC is used.
  crypto::ForceNSSNoDBInit();
  // Load shared libraries before sandbox is raised.
  // NSS is needed to perform hashing for validation caching.
  crypto::LoadNSSLibraries();
#endif
  std::vector<int> empty; // for SendMsg() calls
  size_t prereserved_sandbox_size = CheckReservedAtZero();

  CheckRDebug(argv[0]);

  // Check that IsSandboxed() works. We should not be sandboxed at this point.
  CHECK(!IsSandboxed()) << "Unexpectedly sandboxed!";

  // Send the zygote a message to let it know we are ready to help
  if (!UnixDomainSocket::SendMsg(kNaClZygoteDescriptor,
                                 kNaClHelperStartupAck,
                                 sizeof(kNaClHelperStartupAck), empty)) {
    LOG(ERROR) << "*** send() to zygote failed";
  }

  while (true) {
    int badpid = -1;
    std::vector<int> fds;
    static const unsigned kMaxMessageLength = 2048;
    char buf[kMaxMessageLength];
    const ssize_t msglen = UnixDomainSocket::RecvMsg(kNaClZygoteDescriptor,
                                                     &buf, sizeof(buf), &fds);
    // If the Zygote has started handling requests, we should be sandboxed via
    // the setuid sandbox.
    if (!IsSandboxed()) {
      LOG(ERROR) << "NaCl helper process running without a sandbox!\n"
                 << "Most likely you need to configure your SUID sandbox "
                 << "correctly";
    }
    if (msglen == 0 || (msglen == -1 && errno == ECONNRESET)) {
      // EOF from the browser. Goodbye!
      _exit(0);
    } else if (msglen < 0) {
      LOG(ERROR) << "nacl_helper: receive from zygote failed, errno = "
                 << errno;
    } else if (msglen == sizeof(kNaClForkRequest) - 1 &&
               memcmp(buf, kNaClForkRequest, msglen) == 0) {
      if (kNaClParentFDIndex + 1 == fds.size()) {
        HandleForkRequest(fds, prereserved_sandbox_size);
        continue;  // fork succeeded. Note: child does not return
      } else {
        LOG(ERROR) << "nacl_helper: unexpected number of fds, got "
                   << fds.size();
      }
    } else {
      LOG(ERROR) << "nacl_helper unrecognized request: "
                 << base::GetDoubleQuotedJson(std::string(buf, buf + msglen));
      _exit(-1);
    }
    // if fork fails, send PID=-1 to zygote
    if (!UnixDomainSocket::SendMsg(kNaClZygoteDescriptor, &badpid,
                                   sizeof(badpid), empty)) {
      LOG(ERROR) << "*** send() to zygote failed";
    }
  }
  CHECK(false);  // This routine must not return
}