debuggerd/debuggerd.cpp

/*
 * Copyright 2006, The Android Open Source Project
 *
 * 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 <stdio.h>
#include <errno.h>
#include <signal.h>
#include <pthread.h>
#include <stdarg.h>
#include <fcntl.h>
#include <sys/types.h>
#include <dirent.h>
#include <time.h>

#include <sys/ptrace.h>
#include <sys/wait.h>
#include <elf.h>
#include <sys/stat.h>
#include <sys/poll.h>

#include <selinux/android.h>

#include <log/logger.h>

#include <cutils/sockets.h>
#include <cutils/properties.h>
#include <cutils/debugger.h>

#include <linux/input.h>

#include <private/android_filesystem_config.h>

#include "backtrace.h"
#include "getevent.h"
#include "tombstone.h"
#include "utility.h"

// If the 32 bit executable is compiled on a 64 bit system,
// use the 32 bit socket name.
#if defined(TARGET_IS_64_BIT) && !defined(__LP64__)
#define SOCKET_NAME DEBUGGER32_SOCKET_NAME
#else
#define SOCKET_NAME DEBUGGER_SOCKET_NAME
#endif

struct debugger_request_t {
  debugger_action_t action;
  pid_t pid, tid;
  uid_t uid, gid;
  uintptr_t abort_msg_address;
  int32_t original_si_code;
};

static void wait_for_user_action(const debugger_request_t &request) {
  // Find out the name of the process that crashed.
  char path[64];
  snprintf(path, sizeof(path), "/proc/%d/exe", request.pid);

  char exe[PATH_MAX];
  int count;
  if ((count = readlink(path, exe, sizeof(exe) - 1)) == -1) {
    ALOGE("readlink('%s') failed: %s", path, strerror(errno));
    strlcpy(exe, "unknown", sizeof(exe));
  } else {
    exe[count] = '\0';
  }

  // Explain how to attach the debugger.
  ALOGI("********************************************************\n"
        "* Process %d has been suspended while crashing.\n"
        "* To attach gdbserver for a gdb connection on port 5039\n"
        "* and start gdbclient:\n"
        "*\n"
        "*     gdbclient %s :5039 %d\n"
        "*\n"
        "* Wait for gdb to start, then press the VOLUME DOWN key\n"
        "* to let the process continue crashing.\n"
        "********************************************************",
        request.pid, exe, request.tid);

  // Wait for VOLUME DOWN.
  if (init_getevent() == 0) {
    while (true) {
      input_event e;
      if (get_event(&e, -1) == 0) {
        if (e.type == EV_KEY && e.code == KEY_VOLUMEDOWN && e.value == 0) {
          break;
        }
      }
    }
    uninit_getevent();
  }

  ALOGI("debuggerd resuming process %d", request.pid);
}

static int get_process_info(pid_t tid, pid_t* out_pid, uid_t* out_uid, uid_t* out_gid) {
  char path[64];
  snprintf(path, sizeof(path), "/proc/%d/status", tid);

  FILE* fp = fopen(path, "r");
  if (!fp) {
    return -1;
  }

  int fields = 0;
  char line[1024];
  while (fgets(line, sizeof(line), fp)) {
    size_t len = strlen(line);
    if (len > 6 && !memcmp(line, "Tgid:\t", 6)) {
      *out_pid = atoi(line + 6);
      fields |= 1;
    } else if (len > 5 && !memcmp(line, "Uid:\t", 5)) {
      *out_uid = atoi(line + 5);
      fields |= 2;
    } else if (len > 5 && !memcmp(line, "Gid:\t", 5)) {
      *out_gid = atoi(line + 5);
      fields |= 4;
    }
  }
  fclose(fp);
  return fields == 7 ? 0 : -1;
}

static int selinux_enabled;

/*
 * Corresponds with debugger_action_t enum type in
 * include/cutils/debugger.h.
 */
static const char *debuggerd_perms[] = {
  NULL, /* crash is only used on self, no check applied */
  "dump_tombstone",
  "dump_backtrace"
};

static bool selinux_action_allowed(int s, pid_t tid, debugger_action_t action)
{
  char *scon = NULL, *tcon = NULL;
  const char *tclass = "debuggerd";
  const char *perm;
  bool allowed = false;

  if (selinux_enabled <= 0)
    return true;

  if (action <= 0 || action >= (sizeof(debuggerd_perms)/sizeof(debuggerd_perms[0]))) {
    ALOGE("SELinux:  No permission defined for debugger action %d", action);
    return false;
  }

  perm = debuggerd_perms[action];

  if (getpeercon(s, &scon) < 0) {
    ALOGE("Cannot get peer context from socket\n");
    goto out;
  }

  if (getpidcon(tid, &tcon) < 0) {
    ALOGE("Cannot get context for tid %d\n", tid);
    goto out;
  }

  allowed = (selinux_check_access(scon, tcon, tclass, perm, NULL) == 0);

out:
   freecon(scon);
   freecon(tcon);
   return allowed;
}

static int read_request(int fd, debugger_request_t* out_request) {
  ucred cr;
  socklen_t len = sizeof(cr);
  int status = getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &cr, &len);
  if (status != 0) {
    ALOGE("cannot get credentials");
    return -1;
  }

  ALOGV("reading tid");
  fcntl(fd, F_SETFL, O_NONBLOCK);

  pollfd pollfds[1];
  pollfds[0].fd = fd;
  pollfds[0].events = POLLIN;
  pollfds[0].revents = 0;
  status = TEMP_FAILURE_RETRY(poll(pollfds, 1, 3000));
  if (status != 1) {
    ALOGE("timed out reading tid (from pid=%d uid=%d)\n", cr.pid, cr.uid);
    return -1;
  }

  debugger_msg_t msg;
  memset(&msg, 0, sizeof(msg));
  status = TEMP_FAILURE_RETRY(read(fd, &msg, sizeof(msg)));
  if (status < 0) {
    ALOGE("read failure? %s (pid=%d uid=%d)\n", strerror(errno), cr.pid, cr.uid);
    return -1;
  }
  if (status != sizeof(debugger_msg_t)) {
    ALOGE("invalid crash request of size %d (from pid=%d uid=%d)\n", status, cr.pid, cr.uid);
    return -1;
  }

  out_request->action = static_cast<debugger_action_t>(msg.action);
  out_request->tid = msg.tid;
  out_request->pid = cr.pid;
  out_request->uid = cr.uid;
  out_request->gid = cr.gid;
  out_request->abort_msg_address = msg.abort_msg_address;
  out_request->original_si_code = msg.original_si_code;

  if (msg.action == DEBUGGER_ACTION_CRASH) {
    // Ensure that the tid reported by the crashing process is valid.
    // This check needs to happen again after ptracing the requested thread to prevent a race.
    if (!pid_contains_tid(out_request->pid, out_request->tid)) {
      ALOGE("tid %d does not exist in pid %d. ignoring debug request\n",
            out_request->tid, out_request->pid);
      return -1;
    }
  } else if (cr.uid == 0
            || (cr.uid == AID_SYSTEM && msg.action == DEBUGGER_ACTION_DUMP_BACKTRACE)) {
    // Only root or system can ask us to attach to any process and dump it explicitly.
    // However, system is only allowed to collect backtraces but cannot dump tombstones.
    status = get_process_info(out_request->tid, &out_request->pid,
                              &out_request->uid, &out_request->gid);
    if (status < 0) {
      ALOGE("tid %d does not exist. ignoring explicit dump request\n", out_request->tid);
      return -1;
    }

    if (!selinux_action_allowed(fd, out_request->tid, out_request->action))
      return -1;
  } else {
    // No one else is allowed to dump arbitrary processes.
    return -1;
  }
  return 0;
}

static bool should_attach_gdb(debugger_request_t* request) {
  if (request->action == DEBUGGER_ACTION_CRASH) {
    char value[PROPERTY_VALUE_MAX];
    property_get("debug.db.uid", value, "-1");
    int debug_uid = atoi(value);
    return debug_uid >= 0 && request->uid <= (uid_t)debug_uid;
  }
  return false;
}

#if defined(__LP64__)
static bool is32bit(pid_t tid) {
  char* exeline;
  if (asprintf(&exeline, "/proc/%d/exe", tid) == -1) {
    return false;
  }
  int fd = TEMP_FAILURE_RETRY(open(exeline, O_RDONLY | O_CLOEXEC));
  int saved_errno = errno;
  free(exeline);
  if (fd == -1) {
    ALOGW("Failed to open /proc/%d/exe %s", tid, strerror(saved_errno));
    return false;
  }

  char ehdr[EI_NIDENT];
  ssize_t bytes = TEMP_FAILURE_RETRY(read(fd, &ehdr, sizeof(ehdr)));
  close(fd);
  if (bytes != (ssize_t) sizeof(ehdr) || memcmp(ELFMAG, ehdr, SELFMAG) != 0) {
    return false;
  }
  if (ehdr[EI_CLASS] == ELFCLASS32) {
    return true;
  }
  return false;
}

static void redirect_to_32(int fd, debugger_request_t* request) {
  debugger_msg_t msg;
  memset(&msg, 0, sizeof(msg));
  msg.tid = request->tid;
  msg.action = request->action;

  int sock_fd = socket_local_client(DEBUGGER32_SOCKET_NAME, ANDROID_SOCKET_NAMESPACE_ABSTRACT,
                                    SOCK_STREAM | SOCK_CLOEXEC);
  if (sock_fd < 0) {
    ALOGE("Failed to connect to debuggerd32: %s", strerror(errno));
    return;
  }

  if (TEMP_FAILURE_RETRY(write(sock_fd, &msg, sizeof(msg))) != (ssize_t) sizeof(msg)) {
    ALOGE("Failed to write request to debuggerd32 socket: %s", strerror(errno));
    close(sock_fd);
    return;
  }

  char ack;
  if (TEMP_FAILURE_RETRY(read(sock_fd, &ack, 1)) == -1) {
    ALOGE("Failed to read ack from debuggerd32 socket: %s", strerror(errno));
    close(sock_fd);
    return;
  }

  char buffer[1024];
  ssize_t bytes_read;
  while ((bytes_read = TEMP_FAILURE_RETRY(read(sock_fd, buffer, sizeof(buffer)))) > 0) {
    ssize_t bytes_to_send = bytes_read;
    ssize_t bytes_written;
    do {
      bytes_written = TEMP_FAILURE_RETRY(write(fd, buffer + bytes_read - bytes_to_send,
                                               bytes_to_send));
      if (bytes_written == -1) {
        if (errno == EAGAIN) {
          // Retry the write.
          continue;
        }
        ALOGE("Error while writing data to fd: %s", strerror(errno));
        break;
      }
      bytes_to_send -= bytes_written;
    } while (bytes_written != 0 && bytes_to_send > 0);
    if (bytes_to_send != 0) {
        ALOGE("Failed to write all data to fd: read %zd, sent %zd", bytes_read, bytes_to_send);
        break;
    }
  }
  close(sock_fd);
}
#endif

static void handle_request(int fd) {
  ALOGV("handle_request(%d)\n", fd);

  debugger_request_t request;
  memset(&request, 0, sizeof(request));
  int status = read_request(fd, &request);
  if (!status) {
    ALOGV("BOOM: pid=%d uid=%d gid=%d tid=%d\n",
         request.pid, request.uid, request.gid, request.tid);

#if defined(__LP64__)
    // On 64 bit systems, requests to dump 32 bit and 64 bit tids come
    // to the 64 bit debuggerd. If the process is a 32 bit executable,
    // redirect the request to the 32 bit debuggerd.
    if (is32bit(request.tid)) {
      // Only dump backtrace and dump tombstone requests can be redirected.
      if (request.action == DEBUGGER_ACTION_DUMP_BACKTRACE
          || request.action == DEBUGGER_ACTION_DUMP_TOMBSTONE) {
        redirect_to_32(fd, &request);
      } else {
        ALOGE("debuggerd: Not allowed to redirect action %d to 32 bit debuggerd\n",
              request.action);
      }
      close(fd);
      return;
    }
#endif

    // At this point, the thread that made the request is blocked in
    // a read() call.  If the thread has crashed, then this gives us
    // time to PTRACE_ATTACH to it before it has a chance to really fault.
    //
    // The PTRACE_ATTACH sends a SIGSTOP to the target process, but it
    // won't necessarily have stopped by the time ptrace() returns.  (We
    // currently assume it does.)  We write to the file descriptor to
    // ensure that it can run as soon as we call PTRACE_CONT below.
    // See details in bionic/libc/linker/debugger.c, in function
    // debugger_signal_handler().
    if (!ptrace_attach_thread(request.pid, request.tid)) {
      ALOGE("ptrace attach failed: %s\n", strerror(errno));
    } else {
      // DEBUGGER_ACTION_CRASH requests can come from arbitrary processes and the tid field in
      // the request is sent from the other side. If an attacker can cause a process to be
      // spawned with the pid of their process, they could trick debuggerd into dumping that
      // process by exiting after sending the request. Validate the trusted request.uid/gid
      // to defend against this.
      if (request.action == DEBUGGER_ACTION_CRASH) {
        pid_t pid;
        uid_t uid;
        gid_t gid;
        if (get_process_info(request.tid, &pid, &uid, &gid) != 0) {
          ALOGE("debuggerd: failed to get process info for tid '%d'", request.tid);
          exit(1);
        }

        if (pid != request.pid || uid != request.uid || gid != request.gid) {
          ALOGE(
            "debuggerd: attached task %d does not match request: "
            "expected pid=%d,uid=%d,gid=%d, actual pid=%d,uid=%d,gid=%d",
            request.tid, request.pid, request.uid, request.gid, pid, uid, gid);
          exit(1);
        }
      }

      bool detach_failed = false;
      bool tid_unresponsive = false;
      bool attach_gdb = should_attach_gdb(&request);
      if (TEMP_FAILURE_RETRY(write(fd, "\0", 1)) != 1) {
        ALOGE("failed responding to client: %s\n", strerror(errno));
      } else {
        char* tombstone_path = NULL;

        if (request.action == DEBUGGER_ACTION_CRASH) {
          close(fd);
          fd = -1;
        }

        int total_sleep_time_usec = 0;
        for (;;) {
          int signal = wait_for_sigstop(request.tid, &total_sleep_time_usec, &detach_failed);
          if (signal == -1) {
            tid_unresponsive = true;
            break;
          }

          switch (signal) {
            case SIGSTOP:
              if (request.action == DEBUGGER_ACTION_DUMP_TOMBSTONE) {
                ALOGV("stopped -- dumping to tombstone\n");
                tombstone_path = engrave_tombstone(request.pid, request.tid,
                                                   signal, request.original_si_code,
                                                   request.abort_msg_address, true,
                                                   &detach_failed, &total_sleep_time_usec);
              } else if (request.action == DEBUGGER_ACTION_DUMP_BACKTRACE) {
                ALOGV("stopped -- dumping to fd\n");
                dump_backtrace(fd, -1, request.pid, request.tid, &detach_failed,
                               &total_sleep_time_usec);
              } else {
                ALOGV("stopped -- continuing\n");
                status = ptrace(PTRACE_CONT, request.tid, 0, 0);
                if (status) {
                  ALOGE("ptrace continue failed: %s\n", strerror(errno));
                }
                continue; // loop again
              }
              break;

            case SIGABRT:
            case SIGBUS:
            case SIGFPE:
            case SIGILL:
            case SIGPIPE:
            case SIGSEGV:
#ifdef SIGSTKFLT
            case SIGSTKFLT:
#endif
            case SIGTRAP:
              ALOGV("stopped -- fatal signal\n");
              // Send a SIGSTOP to the process to make all of
              // the non-signaled threads stop moving.  Without
              // this we get a lot of "ptrace detach failed:
              // No such process".
              kill(request.pid, SIGSTOP);
              // don't dump sibling threads when attaching to GDB because it
              // makes the process less reliable, apparently...
              tombstone_path = engrave_tombstone(request.pid, request.tid,
                                                 signal, request.original_si_code,
                                                 request.abort_msg_address, !attach_gdb,
                                                 &detach_failed, &total_sleep_time_usec);
              break;

            default:
              ALOGE("process stopped due to unexpected signal %d\n", signal);
              break;
          }
          break;
        }

        if (request.action == DEBUGGER_ACTION_DUMP_TOMBSTONE) {
          if (tombstone_path) {
            write(fd, tombstone_path, strlen(tombstone_path));
          }
          close(fd);
          fd = -1;
        }
        free(tombstone_path);
      }

      if (!tid_unresponsive) {
        ALOGV("detaching");
        if (attach_gdb) {
          // stop the process so we can debug
          kill(request.pid, SIGSTOP);
        }
        if (ptrace(PTRACE_DETACH, request.tid, 0, 0)) {
          ALOGE("ptrace detach from %d failed: %s", request.tid, strerror(errno));
          detach_failed = true;
        } else if (attach_gdb) {
          // if debug.db.uid is set, its value indicates if we should wait
          // for user action for the crashing process.
          // in this case, we log a message and turn the debug LED on
          // waiting for a gdb connection (for instance)
          wait_for_user_action(request);
        }
      }

      // resume stopped process (so it can crash in peace).
      kill(request.pid, SIGCONT);

      // If we didn't successfully detach, we're still the parent, and the
      // actual parent won't receive a death notification via wait(2).  At this point
      // there's not much we can do about that.
      if (detach_failed) {
        ALOGE("debuggerd committing suicide to free the zombie!\n");
        kill(getpid(), SIGKILL);
      }
    }

  }
  if (fd >= 0) {
    close(fd);
  }
}

static int do_server() {
  // debuggerd crashes can't be reported to debuggerd.
  // Reset all of the crash handlers.
  signal(SIGABRT, SIG_DFL);
  signal(SIGBUS, SIG_DFL);
  signal(SIGFPE, SIG_DFL);
  signal(SIGILL, SIG_DFL);
  signal(SIGSEGV, SIG_DFL);
#ifdef SIGSTKFLT
  signal(SIGSTKFLT, SIG_DFL);
#endif
  signal(SIGTRAP, SIG_DFL);

  // Ignore failed writes to closed sockets
  signal(SIGPIPE, SIG_IGN);

  int logsocket = socket_local_client("logd", ANDROID_SOCKET_NAMESPACE_ABSTRACT, SOCK_DGRAM);
  if (logsocket < 0) {
    logsocket = -1;
  } else {
    fcntl(logsocket, F_SETFD, FD_CLOEXEC);
  }

  struct sigaction act;
  act.sa_handler = SIG_DFL;
  sigemptyset(&act.sa_mask);
  sigaddset(&act.sa_mask,SIGCHLD);
  act.sa_flags = SA_NOCLDWAIT;
  sigaction(SIGCHLD, &act, 0);

  int s = socket_local_server(SOCKET_NAME, ANDROID_SOCKET_NAMESPACE_ABSTRACT, SOCK_STREAM);
  if (s < 0)
    return 1;
  fcntl(s, F_SETFD, FD_CLOEXEC);

  ALOGI("debuggerd: " __DATE__ " " __TIME__ "\n");

  for (;;) {
    sockaddr addr;
    socklen_t alen = sizeof(addr);

    ALOGV("waiting for connection\n");
    int fd = accept(s, &addr, &alen);
    if (fd < 0) {
      ALOGV("accept failed: %s\n", strerror(errno));
      continue;
    }

    fcntl(fd, F_SETFD, FD_CLOEXEC);

    handle_request(fd);
  }
  return 0;
}

static int do_explicit_dump(pid_t tid, bool dump_backtrace) {
  fprintf(stdout, "Sending request to dump task %d.\n", tid);

  if (dump_backtrace) {
    fflush(stdout);
    if (dump_backtrace_to_file(tid, fileno(stdout)) < 0) {
      fputs("Error dumping backtrace.\n", stderr);
      return 1;
    }
  } else {
    char tombstone_path[PATH_MAX];
    if (dump_tombstone(tid, tombstone_path, sizeof(tombstone_path)) < 0) {
      fputs("Error dumping tombstone.\n", stderr);
      return 1;
    }
    fprintf(stderr, "Tombstone written to: %s\n", tombstone_path);
  }
  return 0;
}

static void usage() {
  fputs("Usage: -b [<tid>]\n"
        "  -b dump backtrace to console, otherwise dump full tombstone file\n"
        "\n"
        "If tid specified, sends a request to debuggerd to dump that task.\n"
        "Otherwise, starts the debuggerd server.\n", stderr);
}

int main(int argc, char** argv) {
  union selinux_callback cb;
  if (argc == 1) {
    selinux_enabled = is_selinux_enabled();
    cb.func_log = selinux_log_callback;
    selinux_set_callback(SELINUX_CB_LOG, cb);
    return do_server();
  }

  bool dump_backtrace = false;
  bool have_tid = false;
  pid_t tid = 0;
  for (int i = 1; i < argc; i++) {
    if (!strcmp(argv[i], "-b")) {
      dump_backtrace = true;
    } else if (!have_tid) {
      tid = atoi(argv[i]);
      have_tid = true;
    } else {
      usage();
      return 1;
    }
  }
  if (!have_tid) {
    usage();
    return 1;
  }
  return do_explicit_dump(tid, dump_backtrace);
}