Commit message: Use seccomp to filter out TIOCSTI ioctl

This would otherwise let the sandbox add input to the controlling tty.
CVE: CVE-2019-10063
CWE: CWE-20
Vulnerability description: Flatpak before 1.0.8, 1.1.x and 1.2.x before 1.2.4, and 1.3.x before 1.3.1 allows a sandbox bypass. Flatpak versions since 0.8.1 address CVE-2017-5226 by using a seccomp filter to prevent sandboxed apps from using the TIOCSTI ioctl, which could otherwise be used to inject commands into the controlling terminal so that they would be executed outside the sandbox after the sandboxed app exits. This fix was incomplete: on 64-bit platforms, the seccomp filter could be bypassed by an ioctl request number that has TIOCSTI in its 32 least significant bits and an arbitrary nonzero value in its 32 most significant bits, which the Linux kernel would treat as equivalent to TIOCSTI.

Project: flatpak
File: flatpak-run.c

setup_seccomp (GPtrArray  *argv_array,
               GArray     *fd_array,
               const char *arch,
               gboolean    multiarch,
               gboolean    devel,
               GError    **error)
{
  __attribute__((cleanup (cleanup_seccomp))) scmp_filter_ctx seccomp = NULL;

  /**** BEGIN NOTE ON CODE SHARING
   *
   * There are today a number of different Linux container
   * implementations.  That will likely continue for long into the
   * future.  But we can still try to share code, and it's important
   * to do so because it affects what library and application writers
   * can do, and we should support code portability between different
   * container tools.
   *
   * This syscall blacklist is copied from linux-user-chroot, which was in turn
   * clearly influenced by the Sandstorm.io blacklist.
   *
   * If you make any changes here, I suggest sending the changes along
   * to other sandbox maintainers.  Using the libseccomp list is also
   * an appropriate venue:
   * https://groups.google.com/forum/#!topic/libseccomp
   *
   * A non-exhaustive list of links to container tooling that might
   * want to share this blacklist:
   *
   *  https://github.com/sandstorm-io/sandstorm
   *    in src/sandstorm/supervisor.c++
   *  http://cgit.freedesktop.org/xdg-app/xdg-app/
   *    in common/flatpak-run.c
   *  https://git.gnome.org/browse/linux-user-chroot
   *    in src/setup-seccomp.c
   *
   **** END NOTE ON CODE SHARING
   */
  struct
  {
    int                  scall;
    struct scmp_arg_cmp *arg;
  } syscall_blacklist[] = {
    /* Block dmesg */
    {SCMP_SYS (syslog)},
    /* Useless old syscall */
    {SCMP_SYS (uselib)},
    /* Don't allow you to switch to bsd emulation or whatnot */
    {SCMP_SYS (personality)},
    /* Don't allow disabling accounting */
    {SCMP_SYS (acct)},
    /* 16-bit code is unnecessary in the sandbox, and modify_ldt is a
       historic source of interesting information leaks. */
    {SCMP_SYS (modify_ldt)},
    /* Don't allow reading current quota use */
    {SCMP_SYS (quotactl)},

    /* Don't allow access to the kernel keyring */
    {SCMP_SYS (add_key)},
    {SCMP_SYS (keyctl)},
    {SCMP_SYS (request_key)},

    /* Scary VM/NUMA ops */
    {SCMP_SYS (move_pages)},
    {SCMP_SYS (mbind)},
    {SCMP_SYS (get_mempolicy)},
    {SCMP_SYS (set_mempolicy)},
    {SCMP_SYS (migrate_pages)},

    /* Don't allow subnamespace setups: */
    {SCMP_SYS (unshare)},
    {SCMP_SYS (mount)},
    {SCMP_SYS (pivot_root)},
    {SCMP_SYS (clone), &SCMP_A0 (SCMP_CMP_MASKED_EQ, CLONE_NEWUSER, CLONE_NEWUSER)},
  };

  struct
  {
    int                  scall;
    struct scmp_arg_cmp *arg;
  } syscall_nondevel_blacklist[] = {
    /* Profiling operations; we expect these to be done by tools from outside
     * the sandbox.  In particular perf has been the source of many CVEs.
     */
    {SCMP_SYS (perf_event_open)},
    {SCMP_SYS (ptrace)}
  };
  /* Blacklist all but unix, inet, inet6 and netlink */
  int socket_family_blacklist[] = {
    AF_AX25,
    AF_IPX,
    AF_APPLETALK,
    AF_NETROM,
    AF_BRIDGE,
    AF_ATMPVC,
    AF_X25,
    AF_ROSE,
    AF_DECnet,
    AF_NETBEUI,
    AF_SECURITY,
    AF_KEY,
    AF_NETLINK + 1, /* Last gets CMP_GE, so order is important */
  };
  int i, r;
  glnx_fd_close int fd = -1;
  g_autofree char *fd_str = NULL;
  g_autofree char *path = NULL;

  seccomp = seccomp_init (SCMP_ACT_ALLOW);
  if (!seccomp)
    return flatpak_fail (error, "Initialize seccomp failed");

  if (arch != NULL)
    {
      uint32_t arch_id = 0;
      const uint32_t *extra_arches = NULL;

      if (strcmp (arch, "i386") == 0)
        {
          arch_id = SCMP_ARCH_X86;
        }
      else if (strcmp (arch, "x86_64") == 0)
        {
          arch_id = SCMP_ARCH_X86_64;
          extra_arches = seccomp_x86_64_extra_arches;
        }
      else if (strcmp (arch, "arm") == 0)
        {
          arch_id = SCMP_ARCH_ARM;
        }
#ifdef SCMP_ARCH_AARCH64
      else if (strcmp (arch, "aarch64") == 0)
        {
          arch_id = SCMP_ARCH_AARCH64;
          extra_arches = seccomp_aarch64_extra_arches;
        }
#endif

      /* We only really need to handle arches on multiarch systems.
       * If only one arch is supported the default is fine */
      if (arch_id != 0)
        {
          /* This *adds* the target arch, instead of replacing the
             native one. This is not ideal, because we'd like to only
             allow the target arch, but we can't really disallow the
             native arch at this point, because then bubblewrap
             couldn't continue running. */
          r = seccomp_arch_add (seccomp, arch_id);
          if (r < 0 && r != -EEXIST)
            return flatpak_fail (error, "Failed to add architecture to seccomp filter");

          if (multiarch && extra_arches != NULL)
            {
              unsigned i;
              for (i = 0; extra_arches[i] != 0; i++)
                {
                  r = seccomp_arch_add (seccomp, extra_arches[i]);
                  if (r < 0 && r != -EEXIST)
                    return flatpak_fail (error, "Failed to add multiarch architecture to seccomp filter");
                }
            }
        }
    }

  /* TODO: Should we filter the kernel keyring syscalls in some way?
   * We do want them to be used by desktop apps, but they could also perhaps
   * leak system stuff or secrets from other apps.
   */

  for (i = 0; i < G_N_ELEMENTS (syscall_blacklist); i++)
    {
      int scall = syscall_blacklist[i].scall;
      if (syscall_blacklist[i].arg)
        r = seccomp_rule_add (seccomp, SCMP_ACT_ERRNO (EPERM), scall, 1, *syscall_blacklist[i].arg);
      else
        r = seccomp_rule_add (seccomp, SCMP_ACT_ERRNO (EPERM), scall, 0);
      if (r < 0 && r == -EFAULT /* unknown syscall */)
        return flatpak_fail (error, "Failed to block syscall %d", scall);
    }

  if (!devel)
    {
      for (i = 0; i < G_N_ELEMENTS (syscall_nondevel_blacklist); i++)
        {
          int scall = syscall_nondevel_blacklist[i].scall;
          if (syscall_nondevel_blacklist[i].arg)
            r = seccomp_rule_add (seccomp, SCMP_ACT_ERRNO (EPERM), scall, 1, *syscall_nondevel_blacklist[i].arg);
          else
            r = seccomp_rule_add (seccomp, SCMP_ACT_ERRNO (EPERM), scall, 0);

          if (r < 0 && r == -EFAULT /* unknown syscall */)
            return flatpak_fail (error, "Failed to block syscall %d", scall);
        }
    }

  /* Socket filtering doesn't work on e.g. i386, so ignore failures here
   * However, we need to user seccomp_rule_add_exact to avoid libseccomp doing
   * something else: https://github.com/seccomp/libseccomp/issues/8 */
  for (i = 0; i < G_N_ELEMENTS (socket_family_blacklist); i++)
    {
      int family = socket_family_blacklist[i];
      if (i == G_N_ELEMENTS (socket_family_blacklist) - 1)
        seccomp_rule_add_exact (seccomp, SCMP_ACT_ERRNO (EAFNOSUPPORT), SCMP_SYS (socket), 1, SCMP_A0 (SCMP_CMP_GE, family));
      else
        seccomp_rule_add_exact (seccomp, SCMP_ACT_ERRNO (EAFNOSUPPORT), SCMP_SYS (socket), 1, SCMP_A0 (SCMP_CMP_EQ, family));
    }

  fd = g_file_open_tmp ("flatpak-seccomp-XXXXXX", &path, error);
  if (fd == -1)
    return FALSE;

  unlink (path);

  if (seccomp_export_bpf (seccomp, fd) != 0)
    return flatpak_fail (error, "Failed to export bpf");

  lseek (fd, 0, SEEK_SET);

  fd_str = g_strdup_printf ("%d", fd);
  if (fd_array)
    g_array_append_val (fd_array, fd);

  add_args (argv_array,
            "--seccomp", fd_str,
            NULL);

  fd = -1; /* Don't close on success */

  return TRUE;
}