libminijailpreload.c

/* libminijailpreload.c - preload hack library
 * Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 *
 * This library is preloaded into every program launched by minijail_run().
 * DO NOT EXPORT ANY SYMBOLS FROM THIS LIBRARY. They will replace other symbols
 * in the programs it is preloaded into and cause impossible-to-debug failures.
 * See the minijail0.1 for a design explanation. */

#include "libminijail.h"
#include "libminijail-private.h"

#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <syslog.h>
#include <unistd.h>

static int (*real_main)(int, char **, char **) = NULL;
static void *libc_handle = NULL;

static void die(const char *failed) {
  syslog(LOG_ERR, "libminijail: %s", failed);
  abort();
}

static void unset_in_env(char **envp, const char *name) {
  int i;
  for (i = 0; envp[i]; i++)
    if (!strncmp(envp[i], name, strlen(name)))
      envp[i][0] = '\0';
}

/** @brief Fake main(), spliced in before the real call to main() by
 *         __libc_start_main (see below).
 *  We get serialized commands from our invoking process over an fd specified
 *  by an environment variable (kFdEnvVar). The environment variable is a list
 *  of key=value pairs (see move_commands_to_env); we use them to construct a
 *  jail, then enter it.
 */
static int fake_main(int argc, char **argv, char **envp) {
  char *fd_name = getenv(kFdEnvVar);
  int fd = -1;
  struct minijail *j;
  if (geteuid() != getuid() || getegid() != getgid())
    /* If we didn't do this check, an attacker could set kFdEnvVar for
     * any setuid program that uses libminijail to cause it to get capabilities
     * or a uid it did not expect. */
    /* TODO(wad) why would libminijail interact here? */
    return MINIJAIL_ERR_PRELOAD;
  if (!fd_name)
    return MINIJAIL_ERR_PRELOAD;
  fd = atoi(fd_name);
  if (fd < 0)
    return MINIJAIL_ERR_PRELOAD;

  j = minijail_new();
  if (!j)
    die("preload: out of memory");
  if (minijail_from_fd(fd, j))
    die("preload: failed to parse minijail from parent");
  close(fd);

  /* TODO(ellyjones): this trashes existing preloads, so one can't do:
   * LD_PRELOAD="/tmp/test.so libminijailpreload.so" prog; the descendants of
   * prog will have no LD_PRELOAD set at all. */
  unset_in_env(envp, kLdPreloadEnvVar);
  /* Strip out flags meant for the parent. */
  minijail_preenter(j);
  minijail_enter(j);
  minijail_destroy(j);
  dlclose(libc_handle);
  return real_main(argc, argv, envp);
}

/** @brief LD_PRELOAD override of __libc_start_main.
 *
 *  It is really best if you do not look too closely at this function.
 *  We need to ensure that some of our code runs before the target program (see
 *  the minijail0.1 file in this directory for high-level details about this), and
 *  the only available place to hook is this function, which is normally
 *  responsible for calling main(). Our LD_PRELOAD will overwrite the real
 *  __libc_start_main with this one, so we have to look up the real one from
 *  libc and invoke it with a pointer to the fake main() we'd like to run before
 *  the real main(). We can't just run our setup code *here* because
 *  __libc_start_main is responsible for setting up the C runtime environment,
 *  so we can't rely on things like malloc() being available yet.
 */

int __libc_start_main(int (*main) (int, char **, char **),
                      int argc, char ** ubp_av, void (*init) (void),
                      void (*fini) (void), void (*rtld_fini) (void),
                      void (* stack_end)) {
  void *sym;
  /* This hack is unfortunately required by C99 - casting directly from void* to
   * function pointers is left undefined. See POSIX.1-2003, the Rationale for
   * the specification of dlsym(), and dlsym(3). This deliberately violates
   * strict-aliasing rules, but gcc can't tell. */
  union {
    int (*fn)(int (*main) (int, char **, char **), int argc,
                     char **ubp_av, void (*init) (void), void (*fini) (void),
                     void (*rtld_fini) (void), void (* stack_end));
    void *symval;
  } real_libc_start_main;

  /* We hold this handle for the duration of the real __libc_start_main() and
   * drop it just before calling the real main(). */
  libc_handle = dlopen("libc.so.6", RTLD_NOW);

  if (!libc_handle) {
    syslog(LOG_ERR, "can't dlopen() libc");
    /* We dare not use abort() here because it will run atexit() handlers and
     * try to flush stdio. */
    _exit(1);
  }
  sym = dlsym(libc_handle, "__libc_start_main");
  if (!sym) {
    syslog(LOG_ERR, "can't find the real __libc_start_main()");
    _exit(1);
  }
  real_libc_start_main.symval = sym;
  real_main = main;

  /* Note that we swap fake_main in for main - fake_main knows that it should
   * call real_main after it's done. */
  return real_libc_start_main.fn(fake_main, argc, ubp_av, init, fini, rtld_fini,
                                 stack_end);
}