Add a helper binary to create a basic Secure Computing filter.

The binary uses the 'seccomp' helper library.  The library isn't
needed to load the generated Secure Computing filter.

* configure.ac: Check for 'seccomp' header and library.

* lib-src/seccomp-filter.c: New helper binary to generate a generic
Secure Computing filter for GNU/Linux.

* lib-src/Makefile.in (DONT_INSTALL): Add 'seccomp-filter' helper
binary if possible.
(all): Add Secure Computing filter file if possible.
(seccomp-filter$(EXEEXT)): Compile helper binary.
(seccomp-filter.bpf seccomp-filter.pfc): Generate filter files.

* test/src/emacs-tests.el (emacs-tests/seccomp/allows-stdout)
(emacs-tests/seccomp/forbids-subprocess): New unit tests.

* test/Makefile.in (src/emacs-tests.log): Add dependency on the helper
binary.

1 files changed, 321 insertions, 0 deletions

diff --git a/lib-src/seccomp-filter.c b/lib-src/seccomp-filter.c
new file mode 100644
index 00000000000..9918fb025ef
--- /dev/null
+++ b/lib-src/seccomp-filter.c
@@ -0,0 +1,321 @@
+/* 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);
+}