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080bfdbb8e9f3d23594b101d7f50e7e2f9e4699b
systemd/src/shared/capability.c
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Przemyslaw Kedzierski dd5ae4c36c bus-proxy: cloning smack label 
When dbus client connects to systemd-bus-proxyd through
Unix domain socket proxy takes client's smack label and sets for itself.

It is done before and independent of dropping privileges.

The reason of such soluton is fact that tests of access rights
performed by lsm may take place inside kernel, not only
in userspace of recipient of message.

The bus-proxyd needs CAP_MAC_ADMIN to manipulate its label.

In case of systemd running in system mode, CAP_MAC_ADMIN
should be added to CapabilityBoundingSet in service file of bus-proxyd.

In case of systemd running in user mode ('systemd --user')
it can be achieved by addition
Capabilities=cap_mac_admin=i and SecureBits=keep-caps
to user@.service file
and setting cap_mac_admin+ei on bus-proxyd binary.
2014-12-09 18:23:24 +01:00

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/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
/***
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
#include <assert.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <sys/types.h>
#include <stdarg.h>
#include <ctype.h>
#include <sys/capability.h>
#include <sys/prctl.h>
#include "grp.h"
#include "macro.h"
#include "util.h"
#include "log.h"
#include "fileio.h"
#include "capability.h"
int have_effective_cap(int value) {
_cleanup_cap_free_ cap_t cap;
cap_flag_value_t fv;
cap = cap_get_proc();
if (!cap)
return -errno;
if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
return -errno;
else
return fv == CAP_SET;
}
unsigned long cap_last_cap(void) {
static thread_local unsigned long saved;
static thread_local bool valid = false;
unsigned long p;
if (valid)
return saved;
p = (unsigned long) CAP_LAST_CAP;
if (prctl(PR_CAPBSET_READ, p) < 0) {
/* Hmm, look downwards, until we find one that
* works */
for (p--; p > 0; p --)
if (prctl(PR_CAPBSET_READ, p) >= 0)
break;
} else {
/* Hmm, look upwards, until we find one that doesn't
* work */
for (;; p++)
if (prctl(PR_CAPBSET_READ, p+1) < 0)
break;
}
saved = p;
valid = true;
return p;
}
int capability_bounding_set_drop(uint64_t drop, bool right_now) {
_cleanup_cap_free_ cap_t after_cap = NULL;
cap_flag_value_t fv;
unsigned long i;
int r;
/* If we are run as PID 1 we will lack CAP_SETPCAP by default
* in the effective set (yes, the kernel drops that when
* executing init!), so get it back temporarily so that we can
* call PR_CAPBSET_DROP. */
after_cap = cap_get_proc();
if (!after_cap)
return -errno;
if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
return -errno;
if (fv != CAP_SET) {
_cleanup_cap_free_ cap_t temp_cap = NULL;
static const cap_value_t v = CAP_SETPCAP;
temp_cap = cap_dup(after_cap);
if (!temp_cap) {
r = -errno;
goto finish;
}
if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
r = -errno;
goto finish;
}
if (cap_set_proc(temp_cap) < 0) {
r = -errno;
goto finish;
}
}
for (i = 0; i <= cap_last_cap(); i++) {
if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
cap_value_t v;
/* Drop it from the bounding set */
if (prctl(PR_CAPBSET_DROP, i) < 0) {
r = -errno;
goto finish;
}
v = (cap_value_t) i;
/* Also drop it from the inheritable set, so
* that anything we exec() loses the
* capability for good. */
if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
r = -errno;
goto finish;
}
/* If we shall apply this right now drop it
* also from our own capability sets. */
if (right_now) {
if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
r = -errno;
goto finish;
}
}
}
}
r = 0;
finish:
if (cap_set_proc(after_cap) < 0)
return -errno;
return r;
}
static int drop_from_file(const char *fn, uint64_t drop) {
int r, k;
uint32_t hi, lo;
uint64_t current, after;
char *p;
r = read_one_line_file(fn, &p);
if (r < 0)
return r;
assert_cc(sizeof(hi) == sizeof(unsigned));
assert_cc(sizeof(lo) == sizeof(unsigned));
k = sscanf(p, "%u %u", &lo, &hi);
free(p);
if (k != 2)
return -EIO;
current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
after = current & ~drop;
if (current == after)
return 0;
lo = (unsigned) (after & 0xFFFFFFFFULL);
hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
if (asprintf(&p, "%u %u", lo, hi) < 0)
return -ENOMEM;
r = write_string_file(fn, p);
free(p);
return r;
}
int capability_bounding_set_drop_usermode(uint64_t drop) {
int r;
r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
if (r < 0)
return r;
r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
if (r < 0)
return r;
return r;
}
int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
_cleanup_cap_free_ cap_t d = NULL;
int r;
/* Unfortunately we cannot leave privilege dropping to PID 1
* here, since we want to run as user but want to keep some
* capabilities. Since file capabilities have been introduced
* this cannot be done across exec() anymore, unless our
* binary has the capability configured in the file system,
* which we want to avoid. */
if (setresgid(gid, gid, gid) < 0)
return log_error_errno(errno, "Failed to change group ID: %m");
if (setgroups(0, NULL) < 0)
return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
if (prctl(PR_SET_KEEPCAPS, 1) < 0)
return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
r = setresuid(uid, uid, uid);
if (r < 0)
return log_error_errno(errno, "Failed to change user ID: %m");
if (prctl(PR_SET_KEEPCAPS, 0) < 0)
return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
r = capability_bounding_set_drop(~keep_capabilities, true);
if (r < 0)
return log_error_errno(r, "Failed to drop capabilities: %m");
d = cap_init();
if (!d)
return log_oom();
if (keep_capabilities) {
cap_value_t bits[sizeof(keep_capabilities)*8];
unsigned i, j = 0;
for (i = 0; i < sizeof(keep_capabilities)*8; i++)
if (keep_capabilities & (1ULL << i))
bits[j++] = i;
if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
log_error_errno(errno, "Failed to enable capabilities bits: %m");
return -errno;
}
}
if (cap_set_proc(d) < 0)
return log_error_errno(errno, "Failed to increase capabilities: %m");
return 0;
}
int drop_capability(cap_value_t cv) {
_cleanup_cap_free_ cap_t tmp_cap = NULL;
tmp_cap = cap_get_proc();
if (!tmp_cap)
return -errno;
if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
(cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
(cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
return -errno;
if (cap_set_proc(tmp_cap) < 0)
return -errno;
return 0;
}
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