Commit 1060289f authored by Philipp Stephani's avatar Philipp Stephani
Browse files

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.
parent 2d17e012
Pipeline #10234 passed with stages
in 57 minutes and 25 seconds
......@@ -188,6 +188,7 @@ lib-src/make-docfile
lib-src/make-fingerprint
lib-src/movemail
lib-src/profile
lib-src/seccomp-filter
lib-src/test-distrib
lib-src/update-game-score
nextstep/Cocoa/Emacs.base/Contents/Info.plist
......@@ -301,3 +302,7 @@ nt/emacs.rc
nt/emacsclient.rc
src/gdb.ini
/var/
# Seccomp filter files.
lib-src/seccomp-filter.bpf
lib-src/seccomp-filter.pfc
......@@ -4181,6 +4181,11 @@ AC_SUBST([LIBS_MAIL])
AC_CHECK_HEADERS([linux/seccomp.h], [HAVE_SECCOMP=yes])
LIBSECCOMP=
AC_CHECK_HEADER([seccomp.h],
[AC_CHECK_LIB([seccomp], [seccomp_init], [LIBSECCOMP=-lseccomp])])
AC_SUBST([LIBSECCOMP])
OLD_LIBS=$LIBS
LIBS="$LIB_PTHREAD $LIB_MATH $LIBS"
AC_CHECK_FUNCS(accept4 fchdir gethostname \
......
......@@ -189,6 +189,12 @@ LIB_WSOCK32=@LIB_WSOCK32@
## Extra libraries for etags
LIBS_ETAGS = $(LIB_CLOCK_GETTIME) $(LIB_GETRANDOM)
LIBSECCOMP=@LIBSECCOMP@
ifneq ($(LIBSECCOMP),)
DONT_INSTALL += seccomp-filter$(EXEEXT)
endif
## Extra libraries to use when linking movemail.
LIBS_MOVE = $(LIBS_MAIL) $(KRB4LIB) $(DESLIB) $(KRB5LIB) $(CRYPTOLIB) \
$(COM_ERRLIB) $(LIBHESIOD) $(LIBRESOLV) $(LIB_WSOCK32) $(LIBS_ETAGS)
......@@ -218,6 +224,10 @@ config_h = ../src/config.h $(srcdir)/../src/conf_post.h
all: ${EXE_FILES} ${SCRIPTS}
ifneq ($(LIBSECCOMP),)
all: seccomp-filter.bpf
endif
.PHONY: all need-blessmail maybe-blessmail
LOADLIBES = ../lib/libgnu.a $(LIBS_SYSTEM)
......@@ -400,4 +410,13 @@ update-game-score${EXEEXT}: ${srcdir}/update-game-score.c $(NTLIB) $(config_h)
emacsclient.res: ../nt/emacsclient.rc $(NTINC)/../icons/emacs.ico
$(AM_V_RC)$(WINDRES) -O coff --include-dir=$(NTINC)/.. -o $@ $<
ifneq ($(LIBSECCOMP),)
seccomp-filter$(EXEEXT): $(srcdir)/seccomp-filter.c $(config_h)
$(AM_V_CCLD)$(CC) $(ALL_CFLAGS) $< $(LIBSECCOMP) -o $@
seccomp-filter.bpf seccomp-filter.pfc: seccomp-filter$(EXEEXT)
$(AM_V_GEN)./seccomp-filter$(EXEEXT) \
seccomp-filter.bpf seccomp-filter.pfc
endif
## Makefile ends here.
/* 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);
}
......@@ -276,6 +276,8 @@ $(test_module): $(test_module:${SO}=.c) ../src/emacs-module.h
$(srcdir)/../lib/timespec.c $(srcdir)/../lib/gettime.c
endif
src/emacs-tests.log: ../lib-src/seccomp-filter.c
## Check that there is no 'automated' subdirectory, which would
## indicate an incomplete merge from an older version of Emacs where
## the tests were arranged differently.
......
../../../lib-src/seccomp-filter.bpf
\ No newline at end of file
......@@ -25,7 +25,9 @@
(require 'cl-lib)
(require 'ert)
(require 'ert-x)
(require 'rx)
(require 'subr-x)
(ert-deftest emacs-tests/seccomp/absent-file ()
(skip-unless (string-match-p (rx bow "SECCOMP" eow)
......@@ -128,4 +130,51 @@ to `make-temp-file', which see."
(concat "--seccomp=" filter))
0)))))
(ert-deftest emacs-tests/seccomp/allows-stdout ()
(skip-unless (string-match-p (rx bow "SECCOMP" eow)
system-configuration-features))
(let ((emacs
(expand-file-name invocation-name invocation-directory))
(filter (ert-resource-file "seccomp-filter.bpf"))
(process-environment nil))
(skip-unless (file-executable-p emacs))
(skip-unless (file-readable-p filter))
;; The --seccomp option is processed early, without filename
;; handlers. Therefore remote or quoted filenames wouldn't work.
(should-not (file-remote-p filter))
(cl-callf file-name-unquote filter)
(with-temp-buffer
(let ((status (call-process
emacs nil t nil
"--quick" "--batch"
(concat "--seccomp=" filter)
(format "--eval=%S" '(message "Hi")))))
(ert-info ((format "Process output: %s" (buffer-string)))
(should (eql status 0)))
(should (equal (string-trim (buffer-string)) "Hi"))))))
(ert-deftest emacs-tests/seccomp/forbids-subprocess ()
(skip-unless (string-match-p (rx bow "SECCOMP" eow)
system-configuration-features))
(let ((emacs
(expand-file-name invocation-name invocation-directory))
(filter (ert-resource-file "seccomp-filter.bpf"))
(process-environment nil))
(skip-unless (file-executable-p emacs))
(skip-unless (file-readable-p filter))
;; The --seccomp option is processed early, without filename
;; handlers. Therefore remote or quoted filenames wouldn't work.
(should-not (file-remote-p filter))
(cl-callf file-name-unquote filter)
(with-temp-buffer
(let ((status
(call-process
emacs nil t nil
"--quick" "--batch"
(concat "--seccomp=" filter)
(format "--eval=%S" `(call-process ,emacs nil nil nil
"--version")))))
(ert-info ((format "Process output: %s" (buffer-string)))
(should-not (eql status 0)))))))
;;; emacs-tests.el ends here
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