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Systemd/src/basic/capability-util.c
Ismo Puustinen 755d4b67a4 capabilities: added support for ambient capabilities. 
This patch adds support for ambient capabilities in service files. The
idea with ambient capabilities is that the execed processes can run with
non-root user and get some inherited capabilities, without having any
need to add the capabilities to the executable file.

You need at least Linux 4.3 to use ambient capabilities. SecureBit
keep-caps is automatically added when you use ambient capabilities and
wish to change the user.

An example system service file might look like this:

[Unit]
Description=Service for testing caps

[Service]
ExecStart=/usr/bin/sleep 10000
User=nobody
AmbientCapabilities=CAP_NET_ADMIN CAP_NET_RAW

After starting the service it has these capabilities:

CapInh: 0000000000003000
CapPrm: 0000000000003000
CapEff: 0000000000003000
CapBnd: 0000003fffffffff
CapAmb: 0000000000003000
2016-01-12 12:14:50 +02:00

364 lines
11 KiB
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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 <errno.h>
#include <grp.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/capability.h>
#include <sys/prctl.h>
#include <unistd.h>
#include "alloc-util.h"
#include "capability-util.h"
#include "fileio.h"
#include "log.h"
#include "macro.h"
#include "parse-util.h"
#include "util.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;
_cleanup_free_ char *content = NULL;
unsigned long p = 0;
int r;
if (valid)
return saved;
/* available since linux-3.2 */
r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
if (r >= 0) {
r = safe_atolu(content, &p);
if (r >= 0) {
saved = p;
valid = true;
return p;
}
}
/* fall back to syscall-probing for pre linux-3.2 */
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_update_inherited_set(cap_t caps, uint64_t set) {
unsigned long i;
/* Add capabilities in the set to the inherited caps. Do not apply
* them yet. */
for (i = 0; i < cap_last_cap(); i++) {
if (set & (UINT64_C(1) << i)) {
cap_value_t v;
v = (cap_value_t) i;
/* Make the capability inheritable. */
if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
return -errno;
}
}
return 0;
}
int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
unsigned long i;
_cleanup_cap_free_ cap_t caps = NULL;
/* Add the capabilities to the ambient set. */
if (also_inherit) {
int r;
caps = cap_get_proc();
if (!caps)
return -errno;
r = capability_update_inherited_set(caps, set);
if (r < 0)
return -errno;
if (cap_set_proc(caps) < 0)
return -errno;
}
for (i = 0; i < cap_last_cap(); i++) {
if (set & (UINT64_C(1) << i)) {
/* Add the capability to the ambient set. */
if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
return -errno;
}
}
return 0;
}
int capability_bounding_set_drop(uint64_t keep, 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 (!(keep & (UINT64_C(1) << 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 keep) {
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 & keep;
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, WRITE_STRING_FILE_CREATE);
free(p);
return r;
}
int capability_bounding_set_drop_usermode(uint64_t keep) {
int r;
r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
if (r < 0)
return r;
r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
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;
unsigned i, j = 0;
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");
/* Ensure we keep the permitted caps across the setresuid() */
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");
/* Drop all caps from the bounding set, except the ones we want */
r = capability_bounding_set_drop(keep_capabilities, true);
if (r < 0)
return log_error_errno(r, "Failed to drop capabilities: %m");
/* Now upgrade the permitted caps we still kept to effective caps */
d = cap_init();
if (!d)
return log_oom();
if (keep_capabilities) {
cap_value_t bits[u64log2(keep_capabilities) + 1];
for (i = 0; i < ELEMENTSOF(bits); i++)
if (keep_capabilities & (1ULL << i))
bits[j++] = i;
/* use enough bits */
assert(i == 64 || (keep_capabilities >> i) == 0);
/* don't use too many bits */
assert(keep_capabilities & (1ULL << (i - 1)));
if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
return log_error_errno(errno, "Failed to enable capabilities bits: %m");
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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