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/*
* Copyright (C) 2023 Thomas Weißschuh <thomas@t-8ch.de>
*
* This program 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 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it would 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 this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#include <stddef.h>
#include <stdbool.h>
#include <getopt.h>
#include <linux/unistd.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <linux/audit.h>
#include <sys/prctl.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include "c.h"
#include "exitcodes.h"
#include "nls.h"
#include "bitops.h"
#include "audit-arch.h"
#include "list.h"
#include "xalloc.h"
#include "strutils.h"
#define IS_LITTLE_ENDIAN (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
#define syscall_nr (offsetof(struct seccomp_data, nr))
#define syscall_arch (offsetof(struct seccomp_data, arch))
#define _syscall_arg(n) (offsetof(struct seccomp_data, args[n]))
#define syscall_arg_lower32(n) (_syscall_arg(n) + 4 * !IS_LITTLE_ENDIAN)
#define syscall_arg_upper32(n) (_syscall_arg(n) + 4 * IS_LITTLE_ENDIAN)
static int set_seccomp_filter(const void *prog)
{
#if defined(__NR_seccomp) && defined(SECCOMP_SET_MODE_FILTER) && defined(SECCOMP_FILTER_FLAG_SPEC_ALLOW)
if (!syscall(__NR_seccomp, SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_SPEC_ALLOW, prog))
return 0;
#endif
return prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
}
struct syscall {
const char *const name;
long number;
};
/* When the alias arrays are empty the compiler emits -Wtype-limits warnings.
* Avoid those by going through this function. */
static inline bool lt(size_t a, size_t b)
{
return a < b;
}
static const struct syscall syscalls[] = {
#define UL_SYSCALL(name, nr) { name, nr },
#include "syscalls.h"
#undef UL_SYSCALL
};
static const struct syscall ioctls[] = {
{ "FIOCLEX", FIOCLEX },
};
static void __attribute__((__noreturn__)) usage(void)
{
FILE *out = stdout;
fputs(USAGE_HEADER, out);
fprintf(out, _(" %s [options] -- <command>\n"), program_invocation_short_name);
fputs(USAGE_OPTIONS, out);
fputs(_(" -s, --syscall syscall to block\n"), out);
fputs(_(" -i, --ioctl ioctl to block\n"), out);
fputs(_(" -l, --list list known syscalls\n"), out);
fputs(USAGE_SEPARATOR, out);
fprintf(out, USAGE_HELP_OPTIONS(25));
fprintf(out, USAGE_MAN_TAIL("enosys(1)"));
exit(EXIT_SUCCESS);
}
struct blocked_number {
struct list_head head;
long number;
};
int main(int argc, char **argv)
{
int c;
size_t i;
bool found;
static const struct option longopts[] = {
{ "syscall", required_argument, NULL, 's' },
{ "ioctl", required_argument, NULL, 'i' },
{ "list", no_argument, NULL, 'l' },
{ "list-ioctl", no_argument, NULL, 'm' },
{ "version", no_argument, NULL, 'V' },
{ "help", no_argument, NULL, 'h' },
{ 0 }
};
long blocked_number;
struct blocked_number *blocked;
struct list_head *loop_ctr;
struct list_head blocked_syscalls;
bool blocking_execve = false;
INIT_LIST_HEAD(&blocked_syscalls);
struct list_head blocked_ioctls;
INIT_LIST_HEAD(&blocked_ioctls);
setlocale(LC_ALL, "");
bindtextdomain(PACKAGE, LOCALEDIR);
textdomain(PACKAGE);
while ((c = getopt_long (argc, argv, "+Vhs:i:lm", longopts, NULL)) != -1) {
switch (c) {
case 's':
found = 0;
for (i = 0; lt(i, ARRAY_SIZE(syscalls)); i++) {
if (strcmp(optarg, syscalls[i].name) == 0) {
blocked_number = syscalls[i].number;
found = 1;
break;
}
}
if (!found)
blocked_number = str2num_or_err(
optarg, 10, _("Unknown syscall"), 0, LONG_MAX);
blocked = xmalloc(sizeof(*blocked));
blocked->number = blocked_number;
list_add(&blocked->head, &blocked_syscalls);
if (blocked_number == __NR_execve)
blocking_execve = true;
break;
case 'i':
found = 0;
for (i = 0; lt(i, ARRAY_SIZE(ioctls)); i++) {
if (strcmp(optarg, ioctls[i].name) == 0) {
blocked_number = ioctls[i].number;
found = 1;
break;
}
}
if (!found)
blocked_number = str2num_or_err(
optarg, 10, _("Unknown ioctl"), 0, LONG_MAX);
blocked = xmalloc(sizeof(*blocked));
blocked->number = blocked_number;
list_add(&blocked->head, &blocked_ioctls);
break;
case 'l':
for (i = 0; lt(i, ARRAY_SIZE(syscalls)); i++)
printf("%5ld %s\n", syscalls[i].number, syscalls[i].name);
return EXIT_SUCCESS;
case 'm':
for (i = 0; lt(i, ARRAY_SIZE(ioctls)); i++)
printf("%5ld %s\n", ioctls[i].number, ioctls[i].name);
return EXIT_SUCCESS;
case 'V':
print_version(EXIT_SUCCESS);
case 'h':
usage();
default:
errtryhelp(EXIT_FAILURE);
}
}
if (optind >= argc)
errtryhelp(EXIT_FAILURE);
struct sock_filter filter[BPF_MAXINSNS];
struct sock_filter *f = filter;
#define INSTR(_instruction) \
if (f == &filter[ARRAY_SIZE(filter)]) \
errx(EXIT_FAILURE, _("filter too big")); \
*f++ = (struct sock_filter) _instruction
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arch));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, SECCOMP_ARCH_NATIVE, 1, 0));
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_TRAP));
/* Blocking "execve" normally would also block our own call to
* it and the end of main. To distinguish between our execve
* and the execve to be blocked, compare the environ pointer.
*
* See https://lore.kernel.org/all/CAAnLoWnS74dK9Wq4EQ-uzQ0qCRfSK-dLqh+HCais-5qwDjrVzg@mail.gmail.com/
*/
if (blocking_execve) {
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_nr));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_execve, 0, 5));
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arg_lower32(2)));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (uint64_t)(uintptr_t) environ, 0, 3));
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arg_upper32(2)));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (uint64_t)(uintptr_t) environ >> 32, 0, 1));
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW));
}
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_nr));
list_for_each(loop_ctr, &blocked_syscalls) {
blocked = list_entry(loop_ctr, struct blocked_number, head);
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, blocked->number, 0, 1));
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ERRNO | ENOSYS));
}
if (!list_empty(&blocked_ioctls)) {
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_ioctl, 1, 0));
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW));
list_for_each(loop_ctr, &blocked_ioctls) {
blocked = list_entry(loop_ctr, struct blocked_number, head);
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arg_lower32(1)));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (uint64_t) blocked->number, 0, 3));
INSTR(BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arg_upper32(1)));
INSTR(BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (uint64_t) blocked->number >> 32, 0, 1));
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ERRNO | ENOTTY));
}
}
INSTR(BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW));
struct sock_fprog prog = {
.len = f - filter,
.filter = filter,
};
/* *SET* below will return EINVAL when either the filter is invalid or
* seccomp is not supported. To distinguish those cases do a *GET* here
*/
if (prctl(PR_GET_SECCOMP) == -1 && errno == EINVAL)
err(EXIT_NOTSUPP, _("Seccomp non-functional"));
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
err_nosys(EXIT_FAILURE, _("Could not run prctl(PR_SET_NO_NEW_PRIVS)"));
if (set_seccomp_filter(&prog))
err_nosys(EXIT_FAILURE, _("Could not seccomp filter"));
if (execvp(argv[optind], argv + optind))
err(EXIT_NOTSUPP, _("Could not exec"));
}
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