/* Generate a Secure Computing filter definition file.

Copyright (C) 2020 Free Software Foundation, Inc.

This file is part of GNU Emacs.

GNU Emacs is free software: you can redistribute it and/or modify it under the
terms of the GNU General Public License as published by the Free Software
Foundation, either version 3 of the License, or (at your option) any later
version.

GNU Emacs is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.  See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with GNU
Emacs.  If not, see <https://www.gnu.org/licenses/>.  */

/* This program creates a small Secure Computing filter usable for a typical
minimal Emacs sandbox.  See the man page for `seccomp' for details about Secure
Computing filters.  This program requires the `libseccomp' library.  However,
the resulting filter file requires only a Linux kernel supporting the Secure
Computing extension.

Usage:

  seccomp-filter out.bpf out.pfc

This writes the raw `struct sock_filter' array to out.bpf and a human-readable
representation to out.pfc.  */

#include "config.h"

#include <errno.h>
#include <limits.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>

#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/prctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/futex.h>
#include <fcntl.h>
#include <sched.h>
#include <seccomp.h>
#include <unistd.h>

#include "verify.h"

static ATTRIBUTE_FORMAT_PRINTF (2, 3) _Noreturn void
fail (int error, const char *format, ...)
{
  va_list ap;
  va_start (ap, format);
  if (error == 0)
    vfprintf (stderr, format, ap);
  else
    {
      char buffer[1000];
      vsnprintf (buffer, sizeof buffer, format, ap);
      errno = error;
      perror (buffer);
    }
  va_end (ap);
  fflush (NULL);
  exit (EXIT_FAILURE);
}

/* This binary is trivial, so we use a single global filter context object that
   we release using `atexit'.  */

static scmp_filter_ctx ctx;

static void
release_context (void)
{
  seccomp_release (ctx);
}

/* Wrapper functions and macros for libseccomp functions.  We exit immediately
   upon any error to avoid error checking noise.  */

static void
set_attribute (enum scmp_filter_attr attr, uint32_t value)
{
  int status = seccomp_attr_set (ctx, attr, value);
  if (status < 0)
    fail (-status, "seccomp_attr_set (ctx, %u, %u)", attr, value);
}

/* Like `seccomp_rule_add (ACTION, SYSCALL, ...)', except that you don't have to
   specify the number of comparator arguments, and any failure will exit the
   process.  */

#define RULE(action, syscall, ...)                                      \
  do                                                                    \
    {                                                                   \
      const struct scmp_arg_cmp arg_array[] = {__VA_ARGS__};            \
      enum { arg_cnt = sizeof arg_array / sizeof *arg_array };          \
      int status = seccomp_rule_add_array (ctx, (action), (syscall),    \
                                           arg_cnt, arg_array);         \
      if (status < 0)                                                   \
        fail (-status, "seccomp_rule_add_array (%s, %s, %d, {%s})",	\
	      #action, #syscall, arg_cnt, #__VA_ARGS__);		\
    }                                                                   \
  while (false)

static void
export_filter (const char *file, int (*function) (const scmp_filter_ctx, int),
               const char *name)
{
  int fd = TEMP_FAILURE_RETRY (
    open (file, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY | O_CLOEXEC, 0644));
  if (fd < 0)
    fail (errno, "open %s", file);
  int status = function (ctx, fd);
  if (status < 0)
    fail (-status, "%s", name);
  if (close (fd) != 0)
    fail (errno, "close");
}

#define EXPORT_FILTER(file, function) \
  export_filter ((file), (function), #function)

int
main (int argc, char **argv)
{
  if (argc != 3)
    fail (0, "usage: %s out.bpf out.pfc", argv[0]);

  /* Any unhandled syscall should abort the Emacs process.  */
  ctx = seccomp_init (SCMP_ACT_KILL_PROCESS);
  if (ctx == NULL)
    fail (0, "seccomp_init");
  atexit (release_context);

  /* We want to abort immediately if the architecture is unknown.  */
  set_attribute (SCMP_FLTATR_ACT_BADARCH, SCMP_ACT_KILL_PROCESS);
  set_attribute (SCMP_FLTATR_CTL_NNP, 1);
  set_attribute (SCMP_FLTATR_CTL_TSYNC, 1);
  set_attribute (SCMP_FLTATR_CTL_LOG, 0);

  verify (CHAR_BIT == 8);
  verify (sizeof (int) == 4 && INT_MIN == INT32_MIN && INT_MAX == INT32_MAX);
  verify (sizeof (void *) == 8);
  verify ((uintptr_t) NULL == 0);

  /* Allow a clean exit.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (exit));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (exit_group));

  /* Allow `mmap' and friends.  This is necessary for dynamic loading, reading
     the portable dump file, and thread creation.  We don't allow pages to be
     both writable and executable.  */
  verify (MAP_PRIVATE != 0);
  verify (MAP_SHARED != 0);
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (mmap),
        SCMP_A2_32 (SCMP_CMP_MASKED_EQ, ~(PROT_NONE | PROT_READ | PROT_WRITE)),
        /* Only support known flags.  MAP_DENYWRITE is ignored, but some
           versions of the dynamic loader still use it.  Also allow allocating
           thread stacks.  */
        SCMP_A3_32 (SCMP_CMP_MASKED_EQ,
                    ~(MAP_PRIVATE | MAP_FILE | MAP_ANONYMOUS | MAP_FIXED
                      | MAP_DENYWRITE | MAP_STACK | MAP_NORESERVE),
                    0));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (mmap),
        SCMP_A2_32 (SCMP_CMP_MASKED_EQ, ~(PROT_NONE | PROT_READ | PROT_EXEC)),
        /* Only support known flags.  MAP_DENYWRITE is ignored, but some
           versions of the dynamic loader still use it. */
        SCMP_A3_32 (SCMP_CMP_MASKED_EQ,
                    ~(MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED | MAP_DENYWRITE),
                    0));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (munmap));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (mprotect),
	/* Don't allow making pages executable.  */
        SCMP_A2_32 (SCMP_CMP_MASKED_EQ, ~(PROT_NONE | PROT_READ | PROT_WRITE),
                    0));

  /* Futexes are used everywhere.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (futex),
        SCMP_A1_32 (SCMP_CMP_EQ, FUTEX_WAKE_PRIVATE));

  /* Allow basic dynamic memory management.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (brk));

  /* Allow some status inquiries.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (uname));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getuid));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (geteuid));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getpid));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getpgrp));

  /* Allow operations on open file descriptors.  File descriptors are
     capabilities, and operating on them shouldn't cause security issues.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (read));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (write));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (close));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (lseek));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (dup));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (dup2));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (fstat));

  /* Allow read operations on the filesystem.  If necessary, these should be
     further restricted using mount namespaces.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (access));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (faccessat));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (stat));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (stat64));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (lstat));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (lstat64));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (fstatat64));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (newfstatat));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (readlink));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (readlinkat));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getcwd));

  /* Allow opening files, assuming they are only opened for reading.  */
  verify (O_WRONLY != 0);
  verify (O_RDWR != 0);
  verify (O_CREAT != 0);
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (open),
        SCMP_A1_32 (SCMP_CMP_MASKED_EQ,
                    ~(O_RDONLY | O_BINARY | O_CLOEXEC | O_PATH | O_DIRECTORY),
                    0));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (openat),
        SCMP_A2_32 (SCMP_CMP_MASKED_EQ,
                    ~(O_RDONLY | O_BINARY | O_CLOEXEC | O_PATH | O_DIRECTORY),
                    0));

  /* Allow `tcgetpgrp'.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (ioctl), SCMP_A0_32 (SCMP_CMP_EQ, STDIN_FILENO),
        SCMP_A1_32 (SCMP_CMP_EQ, TIOCGPGRP));

  /* Allow reading (but not setting) file flags.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (fcntl), SCMP_A1_32 (SCMP_CMP_EQ, F_GETFL));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (fcntl64), SCMP_A1_32 (SCMP_CMP_EQ, F_GETFL));

  /* Allow reading random numbers from the kernel.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getrandom));

  /* Changing the umask is uncritical.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (umask));

  /* Allow creation of pipes.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (pipe));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (pipe2));

  /* Allow reading (but not changing) resource limits.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (getrlimit));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (prlimit64),
	SCMP_A0_32 (SCMP_CMP_EQ, 0) /* pid == 0 (current process) */,
        SCMP_A2_64 (SCMP_CMP_EQ, 0) /* new_limit == NULL */);

  /* Block changing resource limits, but don't crash.  */
  RULE (SCMP_ACT_ERRNO (EPERM), SCMP_SYS (prlimit64),
        SCMP_A0_32 (SCMP_CMP_EQ, 0) /* pid == 0 (current process) */,
        SCMP_A2_64 (SCMP_CMP_NE, 0) /* new_limit != NULL */);

  /* Emacs installs signal handlers, which is harmless.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (sigaction));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (rt_sigaction));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (sigprocmask));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (rt_sigprocmask));

  /* Allow timer support.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (timer_create));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (timerfd_create));

  /* Allow thread creation.  See the NOTES section in the manual page for the
     `clone' function.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (clone),
        SCMP_A0_64 (SCMP_CMP_MASKED_EQ,
		    /* Flags needed to create threads.  See create_thread in
		       libc.  */
                    ~(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SYSVSEM
                      | CLONE_SIGHAND | CLONE_THREAD | CLONE_SETTLS
                      | CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID),
                    0));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (sigaltstack));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (set_robust_list));

  /* Allow setting the process name for new threads.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (prctl), SCMP_A0_32 (SCMP_CMP_EQ, PR_SET_NAME));

  /* Allow some event handling functions used by glib.  */
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (eventfd));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (eventfd2));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (wait4));
  RULE (SCMP_ACT_ALLOW, SCMP_SYS (poll));

  /* Don't allow creating sockets (network access would be extremely dangerous),
     but also don't crash.  */
  RULE (SCMP_ACT_ERRNO (EACCES), SCMP_SYS (socket));

  EXPORT_FILTER (argv[1], seccomp_export_bpf);
  EXPORT_FILTER (argv[2], seccomp_export_pfc);
}