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chrony/sys_linux.c
Miroslav Lichvar 434faeecb8 sys_linux: add support for seccomp filters 
The Linux secure computing (seccomp) facility allows a process to
install a filter in the kernel that will allow only specific system
calls to be made. The process is killed when trying to make other system
calls. This is useful to reduce the kernel attack surface and possibly
prevent kernel exploits when the process is compromised.

Use the libseccomp library to add rules and load the filter into the
kernel. Keep a list of system calls that are always allowed after
chronyd is initialized. Restrict arguments that may be passed to the
socket(), setsockopt(), fcntl(), and ioctl() system calls. Arguments
to socketcall(), which is used on some architectures as a multiplexer
instead of separate socket system calls, are not restricted for now.
The mailonchange directive is not allowed as it calls sendmail.

Calls made by the libraries that chronyd is using have to be covered
too. It's difficult to determine which system calls they need as it may
change after an upgrade and it may depend on their configuration (e.g.
resolver in libc). There are also differences between architectures. It
can all break very easily and is therefore disabled by default. It can
be enabled with the new -F option.

This is based on a patch from Andrew Griffiths <agriffit@redhat.com>.
2015-09-04 17:56:51 +02:00

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/*
chronyd/chronyc - Programs for keeping computer clocks accurate.
**********************************************************************
* Copyright (C) Richard P. Curnow 1997-2003
* Copyright (C) John G. Hasler 2009
* Copyright (C) Miroslav Lichvar 2009-2012, 2014
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of version 2 of the GNU General Public License as
* published by the Free Software Foundation.
*
* This program 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 this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*
**********************************************************************
=======================================================================
This is the module specific to the Linux operating system.
*/
#include "config.h"
#include "sysincl.h"
#include <sys/utsname.h>
#if defined(HAVE_SCHED_SETSCHEDULER)
# include <sched.h>
#endif
#if defined(HAVE_MLOCKALL)
# include <sys/mman.h>
#include <sys/resource.h>
#endif
#ifdef FEAT_PRIVDROP
#include <sys/prctl.h>
#include <sys/capability.h>
#include <grp.h>
#endif
#ifdef FEAT_SCFILTER
#include <sys/prctl.h>
#include <seccomp.h>
#ifdef FEAT_PHC
#include <linux/ptp_clock.h>
#endif
#ifdef FEAT_PPS
#include <linux/pps.h>
#endif
#ifdef FEAT_RTC
#include <linux/rtc.h>
#endif
#endif
#include "sys_generic.h"
#include "sys_linux.h"
#include "conf.h"
#include "logging.h"
#include "wrap_adjtimex.h"
/* The threshold for adjtimex maxerror when the kernel sets the UNSYNC flag */
#define UNSYNC_MAXERROR 16.0
/* This is the uncompensated system tick value */
static int nominal_tick;
/* Current tick value */
static int current_delta_tick;
/* The maximum amount by which 'tick' can be biased away from 'nominal_tick'
(sys_adjtimex() in the kernel bounds this to 10%) */
static int max_tick_bias;
/* The kernel USER_HZ constant */
static int hz;
static double dhz; /* And dbl prec version of same for arithmetic */
/* Flag indicating whether adjtimex() can step the clock */
static int have_setoffset;
/* The assumed rate at which the effective frequency and tick values are
updated in the kernel */
static int tick_update_hz;
/* ================================================== */
inline static long
our_round(double x)
{
long y;
if (x > 0.0)
y = x + 0.5;
else
y = x - 0.5;
return y;
}
/* ================================================== */
/* Positive means currently fast of true time, i.e. jump backwards */
static int
apply_step_offset(double offset)
{
if (TMX_ApplyStepOffset(-offset) < 0) {
DEBUG_LOG(LOGF_SysLinux, "adjtimex() failed");
return 0;
}
return 1;
}
/* ================================================== */
/* This call sets the Linux kernel frequency to a given value in parts
per million relative to the nominal running frequency. Nominal is taken to
be tick=10000, freq=0 (for a USER_HZ==100 system, other values otherwise).
The convention is that this is called with a positive argument if the local
clock runs fast when uncompensated. */
static double
set_frequency(double freq_ppm)
{
long required_tick;
double required_freq;
int required_delta_tick;
required_delta_tick = our_round(freq_ppm / dhz);
/* Older kernels (pre-2.6.18) don't apply the frequency offset exactly as
set by adjtimex() and a scaling constant (that depends on the internal
kernel HZ constant) would be needed to compensate for the error. Because
chronyd is closed loop it doesn't matter much if we don't scale the
required frequency, but we want to prevent thrashing between two states
when the system's frequency error is close to a multiple of USER_HZ. With
USER_HZ <= 250, the maximum frequency adjustment of 500 ppm overlaps at
least two ticks and we can stick to the current tick if it's next to the
required tick. */
if (hz <= 250 && (required_delta_tick + 1 == current_delta_tick ||
required_delta_tick - 1 == current_delta_tick)) {
required_delta_tick = current_delta_tick;
}
required_freq = -(freq_ppm - dhz * required_delta_tick);
required_tick = nominal_tick - required_delta_tick;
if (TMX_SetFrequency(&required_freq, required_tick) < 0) {
LOG_FATAL(LOGF_SysLinux, "adjtimex failed for set_frequency, freq_ppm=%10.4e required_freq=%10.4e required_tick=%ld",
freq_ppm, required_freq, required_tick);
}
current_delta_tick = required_delta_tick;
return dhz * current_delta_tick - required_freq;
}
/* ================================================== */
/* Read the ppm frequency from the kernel */
static double
read_frequency(void)
{
long tick;
double freq;
if (TMX_GetFrequency(&freq, &tick) < 0) {
LOG_FATAL(LOGF_SysLinux, "adjtimex() failed");
}
current_delta_tick = nominal_tick - tick;
return dhz * current_delta_tick - freq;
}
/* ================================================== */
static void
set_leap(int leap)
{
int applied;
applied = 0;
if (!leap && TMX_GetLeapApplied(&applied) < 0) {
LOG_FATAL(LOGF_SysLinux, "adjtimex() failed in set_leap");
}
if (TMX_SetLeap(leap) < 0) {
LOG_FATAL(LOGF_SysLinux, "adjtimex() failed in set_leap");
}
LOG(LOGS_INFO, LOGF_SysLinux, "System clock status %s leap second",
leap ? (leap > 0 ? "set to insert" : "set to delete") :
(applied ? "reset after" : "set to not insert/delete"));
}
/* ================================================== */
static void
set_sync_status(int synchronised, double est_error, double max_error)
{
if (synchronised) {
if (est_error > UNSYNC_MAXERROR)
est_error = UNSYNC_MAXERROR;
if (max_error >= UNSYNC_MAXERROR) {
max_error = UNSYNC_MAXERROR;
synchronised = 0;
}
} else {
est_error = max_error = UNSYNC_MAXERROR;
}
/* Clear the UNSYNC flag only if rtcsync is enabled */
if (!CNF_GetRtcSync())
synchronised = 0;
TMX_SetSync(synchronised, est_error, max_error);
}
/* ================================================== */
/* Estimate the value of USER_HZ given the value of txc.tick that chronyd finds when
* it starts. The only credible values are 100 (Linux/x86) or powers of 2.
* Also, the bounds checking inside the kernel's adjtimex system call enforces
* a +/- 10% movement of tick away from the nominal value 1e6/USER_HZ. */
static int
guess_hz(int tick)
{
int i, tick_lo, tick_hi, ihz;
double tick_nominal;
/* Pick off the hz=100 case first */
if (tick >= 9000 && tick <= 11000) {
return 100;
}
for (i=4; i<16; i++) { /* surely 16 .. 32768 is a wide enough range? */
ihz = 1 << i;
tick_nominal = 1.0e6 / (double) ihz;
tick_lo = (int)(0.5 + tick_nominal*2.0/3.0);
tick_hi = (int)(0.5 + tick_nominal*4.0/3.0);
if (tick_lo < tick && tick <= tick_hi) {
return ihz;
}
}
/* oh dear. doomed. */
return 0;
}
/* ================================================== */
static int
get_hz(void)
{
#ifdef _SC_CLK_TCK
int hz;
if ((hz = sysconf(_SC_CLK_TCK)) < 1)
return 0;
return hz;
#else
return 0;
#endif
}
/* ================================================== */
static int
kernelvercmp(int major1, int minor1, int patch1,
int major2, int minor2, int patch2)
{
if (major1 != major2)
return major1 - major2;
if (minor1 != minor2)
return minor1 - minor2;
return patch1 - patch2;
}
/* ================================================== */
/* Compute the scaling to use on any frequency we set, according to
the vintage of the Linux kernel being used. */
static void
get_version_specific_details(void)
{
int major, minor, patch;
long tick;
double freq;
struct utsname uts;
hz = get_hz();
if (!hz) {
if (TMX_GetFrequency(&freq, &tick) < 0)
LOG_FATAL(LOGF_SysLinux, "adjtimex() failed");
hz = guess_hz(tick);
if (!hz)
LOG_FATAL(LOGF_SysLinux, "Can't determine hz from tick %ld", tick);
}
dhz = (double) hz;
nominal_tick = (1000000L + (hz/2))/hz; /* Mirror declaration in kernel */
max_tick_bias = nominal_tick / 10;
/* We can't reliably detect the internal kernel HZ, it may not even be fixed
(CONFIG_NO_HZ aka tickless), assume the lowest commonly used fixed rate */
tick_update_hz = 100;
if (uname(&uts) < 0) {
LOG_FATAL(LOGF_SysLinux, "Cannot uname(2) to get kernel version, sorry.");
}
patch = 0;
if (sscanf(uts.release, "%d.%d.%d", &major, &minor, &patch) < 2) {
LOG_FATAL(LOGF_SysLinux, "Cannot read information from uname, sorry");
}
DEBUG_LOG(LOGF_SysLinux, "Linux kernel major=%d minor=%d patch=%d", major, minor, patch);
if (kernelvercmp(major, minor, patch, 2, 2, 0) < 0) {
LOG_FATAL(LOGF_SysLinux, "Kernel version not supported, sorry.");
}
if (kernelvercmp(major, minor, patch, 2, 6, 27) >= 0 &&
kernelvercmp(major, minor, patch, 2, 6, 33) < 0) {
/* Tickless kernels before 2.6.33 accumulated ticks only in
half-second intervals */
tick_update_hz = 2;
}
/* ADJ_SETOFFSET support */
if (kernelvercmp(major, minor, patch, 2, 6, 39) < 0) {
have_setoffset = 0;
} else {
have_setoffset = 1;
}
DEBUG_LOG(LOGF_SysLinux, "hz=%d nominal_tick=%d max_tick_bias=%d",
hz, nominal_tick, max_tick_bias);
}
/* ================================================== */
/* Initialisation code for this module */
void
SYS_Linux_Initialise(void)
{
get_version_specific_details();
if (TMX_ResetOffset() < 0) {
LOG_FATAL(LOGF_SysLinux, "adjtimex() failed");
}
if (have_setoffset && TMX_TestStepOffset() < 0) {
LOG(LOGS_INFO, LOGF_SysLinux, "adjtimex() doesn't support ADJ_SETOFFSET");
have_setoffset = 0;
}
SYS_Generic_CompleteFreqDriver(1.0e6 * max_tick_bias / nominal_tick,
1.0 / tick_update_hz,
read_frequency, set_frequency,
have_setoffset ? apply_step_offset : NULL,
set_leap, set_sync_status);
}
/* ================================================== */
/* Finalisation code for this module */
void
SYS_Linux_Finalise(void)
{
SYS_Generic_Finalise();
}
/* ================================================== */
#ifdef FEAT_PRIVDROP
void
SYS_Linux_DropRoot(uid_t uid, gid_t gid)
{
cap_t cap;
if (prctl(PR_SET_KEEPCAPS, 1)) {
LOG_FATAL(LOGF_SysLinux, "prctl() failed");
}
if (setgroups(0, NULL)) {
LOG_FATAL(LOGF_SysLinux, "setgroups() failed");
}
if (setgid(gid)) {
LOG_FATAL(LOGF_SysLinux, "setgid(%d) failed", gid);
}
if (setuid(uid)) {
LOG_FATAL(LOGF_SysLinux, "setuid(%d) failed", uid);
}
if ((cap = cap_from_text("cap_net_bind_service,cap_sys_time=ep")) == NULL) {
LOG_FATAL(LOGF_SysLinux, "cap_from_text() failed");
}
if (cap_set_proc(cap)) {
LOG_FATAL(LOGF_SysLinux, "cap_set_proc() failed");
}
cap_free(cap);
DEBUG_LOG(LOGF_SysLinux, "Root dropped to uid %d gid %d", uid, gid);
}
#endif
/* ================================================== */
#ifdef FEAT_SCFILTER
static
void check_seccomp_applicability(void)
{
int mail_enabled;
double mail_threshold;
char *mail_user;
CNF_GetMailOnChange(&mail_enabled, &mail_threshold, &mail_user);
if (mail_enabled)
LOG_FATAL(LOGF_SysLinux, "mailonchange directive cannot be used with -F enabled");
}
/* ================================================== */
void
SYS_Linux_EnableSystemCallFilter(int level)
{
const int syscalls[] = {
/* Clock */
SCMP_SYS(adjtimex), SCMP_SYS(gettimeofday), SCMP_SYS(settimeofday),
SCMP_SYS(time),
/* Process */
SCMP_SYS(clone), SCMP_SYS(exit), SCMP_SYS(exit_group),
SCMP_SYS(rt_sigreturn), SCMP_SYS(sigreturn),
/* Memory */
SCMP_SYS(brk), SCMP_SYS(madvise), SCMP_SYS(mmap), SCMP_SYS(mmap2),
SCMP_SYS(mprotect), SCMP_SYS(munmap), SCMP_SYS(shmdt),
/* Filesystem */
SCMP_SYS(chmod), SCMP_SYS(chown), SCMP_SYS(chown32), SCMP_SYS(fstat),
SCMP_SYS(fstat64), SCMP_SYS(lseek), SCMP_SYS(rename), SCMP_SYS(stat),
SCMP_SYS(stat64), SCMP_SYS(unlink),
/* Socket */
SCMP_SYS(bind), SCMP_SYS(connect), SCMP_SYS(getsockname),
SCMP_SYS(recvfrom), SCMP_SYS(recvmsg), SCMP_SYS(sendmmsg),
SCMP_SYS(sendmsg), SCMP_SYS(sendto),
/* TODO: check socketcall arguments */
SCMP_SYS(socketcall),
/* General I/O */
SCMP_SYS(_newselect), SCMP_SYS(close), SCMP_SYS(open), SCMP_SYS(pipe),
SCMP_SYS(poll), SCMP_SYS(read), SCMP_SYS(futex), SCMP_SYS(select),
SCMP_SYS(set_robust_list), SCMP_SYS(write),
};
const int socket_domains[] = {
AF_NETLINK, AF_UNIX, AF_INET,
#ifdef FEAT_IPV6
AF_INET6,
#endif
};
const static int socket_options[][2] = {
{ SOL_IP, IP_PKTINFO },
#ifdef FEAT_IPV6
{ SOL_IPV6, IPV6_V6ONLY }, { SOL_IPV6, IPV6_RECVPKTINFO },
#endif
{ SOL_SOCKET, SO_BROADCAST }, { SOL_SOCKET, SO_REUSEADDR },
{ SOL_SOCKET, SO_TIMESTAMP },
};
const static int fcntls[] = { F_GETFD, F_SETFD };
const static unsigned long ioctls[] = {
FIONREAD,
#ifdef FEAT_PPS
PTP_SYS_OFFSET,
#endif
#ifdef FEAT_PPS
PPS_FETCH,
#endif
#ifdef FEAT_RTC
RTC_RD_TIME, RTC_SET_TIME, RTC_UIE_ON, RTC_UIE_OFF,
#endif
};
scmp_filter_ctx *ctx;
int i;
/* Check if the chronyd configuration is supported */
check_seccomp_applicability();
ctx = seccomp_init(level > 0 ? SCMP_ACT_KILL : SCMP_ACT_TRAP);
if (ctx == NULL)
LOG_FATAL(LOGF_SysLinux, "Failed to initialize seccomp");
/* Add system calls that are always allowed */
for (i = 0; i < (sizeof (syscalls) / sizeof (*syscalls)); i++) {
if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, syscalls[i], 0) < 0)
goto add_failed;
}
/* Allow sockets to be created only in selected domains */
for (i = 0; i < sizeof (socket_domains) / sizeof (*socket_domains); i++) {
if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket), 1,
SCMP_A0(SCMP_CMP_EQ, socket_domains[i])) < 0)
goto add_failed;
}
/* Allow setting only selected sockets options */
for (i = 0; i < sizeof (socket_options) / sizeof (*socket_options); i++) {
if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt), 3,
SCMP_A1(SCMP_CMP_EQ, socket_options[i][0]),
SCMP_A2(SCMP_CMP_EQ, socket_options[i][1]),
SCMP_A4(SCMP_CMP_LE, sizeof (int))) < 0)
goto add_failed;
}
/* Allow only selected fcntl calls */
for (i = 0; i < sizeof (fcntls) / sizeof (*fcntls); i++) {
if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl), 1,
SCMP_A1(SCMP_CMP_EQ, fcntls[i])) < 0 ||
seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl64), 1,
SCMP_A1(SCMP_CMP_EQ, fcntls[i])) < 0)
goto add_failed;
}
/* Allow only selected ioctls */
for (i = 0; i < sizeof (ioctls) / sizeof (*ioctls); i++) {
if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(ioctl), 1,
SCMP_A1(SCMP_CMP_EQ, ioctls[i])) < 0)
goto add_failed;
}
if (seccomp_load(ctx) < 0)
LOG(LOGS_INFO, LOGF_SysLinux, "Failed to load seccomp rules");
LOG(LOGS_INFO, LOGF_SysLinux, "Loaded seccomp filter");
seccomp_release(ctx);
return;
add_failed:
LOG_FATAL(LOGF_SysLinux, "Failed to add seccomp rules");
}
#endif
/* ================================================== */
#if defined(HAVE_SCHED_SETSCHEDULER)
/* Install SCHED_FIFO real-time scheduler with specified priority */
void SYS_Linux_SetScheduler(int SchedPriority)
{
int pmax, pmin;
struct sched_param sched;
if (SchedPriority < 1 || SchedPriority > 99) {
LOG_FATAL(LOGF_SysLinux, "Bad scheduler priority: %d", SchedPriority);
} else {
sched.sched_priority = SchedPriority;
pmax = sched_get_priority_max(SCHED_FIFO);
pmin = sched_get_priority_min(SCHED_FIFO);
if ( SchedPriority > pmax ) {
sched.sched_priority = pmax;
}
else if ( SchedPriority < pmin ) {
sched.sched_priority = pmin;
}
if ( sched_setscheduler(0, SCHED_FIFO, &sched) == -1 ) {
LOG(LOGS_ERR, LOGF_SysLinux, "sched_setscheduler() failed");
}
else {
DEBUG_LOG(LOGF_SysLinux, "Enabled SCHED_FIFO with priority %d",
sched.sched_priority);
}
}
}
#endif /* HAVE_SCHED_SETSCHEDULER */
#if defined(HAVE_MLOCKALL)
/* Lock the process into RAM so that it will never be swapped out */
void SYS_Linux_MemLockAll(int LockAll)
{
struct rlimit rlim;
if (LockAll == 1 ) {
/* Make sure that we will be able to lock all the memory we need */
/* even after dropping privileges. This does not actually reaerve any memory */
rlim.rlim_max = RLIM_INFINITY;
rlim.rlim_cur = RLIM_INFINITY;
if (setrlimit(RLIMIT_MEMLOCK, &rlim) < 0) {
LOG(LOGS_ERR, LOGF_SysLinux, "setrlimit() failed: not locking into RAM");
}
else {
if (mlockall(MCL_CURRENT|MCL_FUTURE) < 0) {
LOG(LOGS_ERR, LOGF_SysLinux, "mlockall() failed");
}
else {
DEBUG_LOG(LOGF_SysLinux, "Successfully locked into RAM");
}
}
}
}
#endif /* HAVE_MLOCKALL */
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