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systemd/src/basic/capability-util.c

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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <linux/prctl.h>
#include <stdatomic.h>
#include <stdio.h>
#include <sys/prctl.h>
#include <sys/syscall.h>
#include <unistd.h>
#include "alloc-util.h"
#include "bitfield.h"
#include "capability-list.h"
#include "capability-util.h"
#include "errno-util.h"
#include "fd-util.h"
#include "fileio.h"
#include "log.h"
#include "parse-util.h"
#include "pidref.h"
#include "process-util.h"
#include "stat-util.h"
#include "user-util.h"
int capability_get(CapabilityQuintet *ret) {
assert(ret);
struct __user_cap_header_struct hdr = {
.version = _LINUX_CAPABILITY_VERSION_3,
.pid = getpid_cached(),
};
assert_cc(_LINUX_CAPABILITY_U32S_3 == 2);
struct __user_cap_data_struct data[_LINUX_CAPABILITY_U32S_3];
if (syscall(SYS_capget, &hdr, data) < 0)
return -errno;
*ret = (CapabilityQuintet) {
.effective = (uint64_t) data[0].effective | ((uint64_t) data[1].effective << 32),
.bounding = UINT64_MAX,
.inheritable = (uint64_t) data[0].inheritable | ((uint64_t) data[1].inheritable << 32),
.permitted = (uint64_t) data[0].permitted | ((uint64_t) data[1].permitted << 32),
.ambient = UINT64_MAX,
};
return 0;
}
static int capability_apply(const CapabilityQuintet *q) {
assert(q);
struct __user_cap_header_struct hdr = {
.version = _LINUX_CAPABILITY_VERSION_3,
.pid = getpid_cached(),
};
struct __user_cap_data_struct data[_LINUX_CAPABILITY_U32S_3] = {
{
.effective = (uint32_t) (q->effective & UINT32_MAX),
.inheritable = (uint32_t) (q->inheritable & UINT32_MAX),
.permitted = (uint32_t) (q->permitted & UINT32_MAX),
},
{
.effective = (uint32_t) (q->effective >> 32),
.inheritable = (uint32_t) (q->inheritable >> 32),
.permitted = (uint32_t) (q->permitted >> 32),
},
};
return RET_NERRNO(syscall(SYS_capset, &hdr, data));
}
unsigned cap_last_cap(void) {
static atomic_int saved = INT_MAX;
int r, c;
c = saved;
if (c != INT_MAX)
return c;
/* Available since linux-3.2 */
_cleanup_free_ char *content = NULL;
r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
if (r < 0)
log_debug_errno(r, "Failed to read /proc/sys/kernel/cap_last_cap, ignoring: %m");
else {
r = safe_atoi(content, &c);
if (r < 0)
log_debug_errno(r, "Failed to parse /proc/sys/kernel/cap_last_cap, ignoring: %m");
else {
if (c > CAP_LIMIT) /* Safety for the future: if one day the kernel learns more than
* 64 caps, then we are in trouble (since we, as much userspace
* and kernel space store capability masks in uint64_t types). We
* also want to use UINT64_MAX as marker for "unset". Hence let's
* hence protect ourselves against that and always cap at 62 for
* now. */
c = CAP_LIMIT;
saved = c;
return c;
}
}
/* Fall back to syscall-probing for pre linux-3.2, or where /proc/ is not mounted */
unsigned long p = (unsigned long) MIN(CAP_LAST_CAP, CAP_LIMIT);
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 < CAP_LIMIT; p++)
if (prctl(PR_CAPBSET_READ, p+1) < 0)
break;
}
c = (int) p;
saved = c;
return c;
}
int have_effective_cap(unsigned cap) {
CapabilityQuintet q;
int r;
assert(cap <= CAP_LIMIT);
r = capability_get(&q);
if (r < 0)
return r;
return BIT_SET(q.effective, cap);
}
int have_inheritable_cap(unsigned cap) {
CapabilityQuintet q;
int r;
assert(cap <= CAP_LIMIT);
r = capability_get(&q);
if (r < 0)
return r;
return BIT_SET(q.inheritable, cap);
}
int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
int r;
/* Remove capabilities requested in ambient set, but not in the bounding set */
for (unsigned i = 0; i <= cap_last_cap(); i++) {
if (!BIT_SET(set, i))
continue;
if (prctl(PR_CAPBSET_READ, (unsigned long) i) != 1) {
log_debug("Ambient capability %s requested but missing from bounding set, suppressing automatically.",
capability_to_name(i));
CLEAR_BIT(set, i);
}
}
/* Add the capabilities to the ambient set (an possibly also the inheritable set) */
if (also_inherit) {
CapabilityQuintet q;
r = capability_get(&q);
if (r < 0)
return r;
q.inheritable = set;
r = capability_apply(&q);
if (r < 0)
return r;
}
for (unsigned i = 0; i <= cap_last_cap(); i++)
if (BIT_SET(set, i)) {
/* Add the capability to the ambient set. */
if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
return -errno;
} else {
/* Drop the capability so we don't inherit capabilities we didn't ask for. */
r = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, i, 0, 0);
if (r < 0)
return -errno;
if (r > 0)
if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_LOWER, i, 0, 0) < 0)
return -errno;
}
return 0;
}
int capability_gain_cap_setpcap(void) {
CapabilityQuintet q;
int r;
r = capability_get(&q);
if (r < 0)
return r;
if (BIT_SET(q.effective, CAP_SETPCAP))
return 1; /* We already have capability. */
SET_BIT(q.effective, CAP_SETPCAP);
r = capability_apply(&q);
if (r < 0) {
/* If we didn't manage to acquire the CAP_SETPCAP bit, we continue anyway, after all this
* just means we'll fail later, when we actually intend to drop some capabilities or try to
* set securebits. */
log_debug_errno(r, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
return 0;
}
return 1; /* acquired */
}
int capability_bounding_set_drop(uint64_t keep, bool right_now) {
int k, 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. */
CapabilityQuintet q;
r = capability_get(&q);
if (r < 0)
return r;
CapabilityQuintet saved = q;
r = capability_gain_cap_setpcap();
if (r < 0)
return r;
for (unsigned i = 0; i <= cap_last_cap(); i++) {
if (BIT_SET(keep, i))
continue;
/* Drop it from the bounding set */
if (prctl(PR_CAPBSET_DROP, i) < 0) {
r = -errno;
/* If dropping the capability failed, let's see if we didn't have it in the first
* place. If so, continue anyway, as dropping a capability we didn't have in the
* first place doesn't really matter anyway. */
if (prctl(PR_CAPBSET_READ, i) != 0)
goto finish;
}
/* Also drop it from the inheritable set, so that anything we exec() loses the capability for
* good. */
CLEAR_BIT(q.inheritable, i);
/* If we shall apply this right now drop it also from our own capability sets. */
if (right_now) {
CLEAR_BIT(q.effective, i);
CLEAR_BIT(q.permitted, i);
}
}
r = 0;
finish:
k = capability_apply(&q);
if (k < 0)
/* If there are no actual changes anyway then let's ignore this error. */
if (!capability_quintet_equal(&q, &saved))
return k;
return r;
}
static int drop_from_file(const char *fn, uint64_t keep) {
_cleanup_free_ char *p = NULL;
uint64_t current, after;
uint32_t hi, lo;
int r, k;
r = read_one_line_file(fn, &p);
if (r < 0)
return r;
k = sscanf(p, "%" PRIu32 " %" PRIu32, &lo, &hi);
if (k != 2)
return -EIO;
current = (uint64_t) lo | ((uint64_t) hi << 32);
after = current & keep;
if (current == after)
return 0;
lo = after & UINT32_MAX;
hi = (after >> 32) & UINT32_MAX;
return write_string_filef(fn, 0, "%" PRIu32 " %" PRIu32, lo, hi);
}
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) {
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");
r = maybe_setgroups(/* size= */ 0, /* list= */ NULL);
if (r < 0)
return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
/* Ensure we keep the permitted caps across the setresuid(). Note that we do this even if we actually
* don't want to keep any capabilities, since we want to be able to drop them from the bounding set
* too, and we can only do that if we have capabilities. */
if (prctl(PR_SET_KEEPCAPS, 1) < 0)
return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
if (setresuid(uid, uid, uid) < 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 (as well as the inheritable/permitted/effective sets), except
* the ones we want to keep */
r = capability_bounding_set_drop(keep_capabilities, /* right_now= */ 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 */
if (keep_capabilities != 0) {
CapabilityQuintet q = {
.effective = keep_capabilities,
.permitted = keep_capabilities,
};
r = capability_apply(&q);
if (r < 0)
return log_error_errno(r, "Failed to increase capabilities: %m");
}
return 0;
}
static int change_capability(unsigned cap, bool b) {
CapabilityQuintet q;
int r;
assert(cap <= CAP_LIMIT);
r = capability_get(&q);
if (r < 0)
return r;
if (b) {
SET_BIT(q.effective, cap);
SET_BIT(q.permitted, cap);
SET_BIT(q.inheritable, cap);
} else {
CLEAR_BIT(q.effective, cap);
CLEAR_BIT(q.permitted, cap);
CLEAR_BIT(q.inheritable, cap);
}
return capability_apply(&q);
}
int drop_capability(unsigned cap) {
return change_capability(cap, false);
}
int keep_capability(unsigned cap) {
return change_capability(cap, true);
}
bool capability_quintet_mangle(CapabilityQuintet *q) {
uint64_t combined, drop = 0;
assert(q);
combined = q->effective | q->bounding | q->inheritable | q->permitted | q->ambient;
for (unsigned i = 0; i <= cap_last_cap(); i++) {
if (!BIT_SET(combined, i))
continue;
if (prctl(PR_CAPBSET_READ, (unsigned long) i) > 0)
continue;
SET_BIT(drop, i);
log_debug("Dropping capability not in the current bounding set: %s", capability_to_name(i));
}
q->effective &= ~drop;
q->bounding &= ~drop;
q->inheritable &= ~drop;
q->permitted &= ~drop;
q->ambient &= ~drop;
return drop != 0; /* Let the caller know we changed something */
}
int capability_quintet_enforce(const CapabilityQuintet *q) {
CapabilityQuintet c;
bool modified = false;
int r;
if (q->ambient != CAP_MASK_UNSET ||
q->inheritable != CAP_MASK_UNSET ||
q->permitted != CAP_MASK_UNSET ||
q->effective != CAP_MASK_UNSET) {
r = capability_get(&c);
if (r < 0)
return r;
}
if (q->ambient != CAP_MASK_UNSET) {
/* In order to raise the ambient caps set we first need to raise the matching
* inheritable + permitted cap */
if (!FLAGS_SET(c.permitted, q->ambient) ||
!FLAGS_SET(c.inheritable, q->ambient)) {
c.permitted |= q->ambient;
c.inheritable |= q->ambient;
r = capability_apply(&c);
if (r < 0)
return r;
}
r = capability_ambient_set_apply(q->ambient, /* also_inherit= */ false);
if (r < 0)
return r;
}
if (q->inheritable != CAP_MASK_UNSET || q->permitted != CAP_MASK_UNSET || q->effective != CAP_MASK_UNSET) {
if (!FLAGS_SET(c.effective, q->effective) ||
!FLAGS_SET(c.permitted, q->permitted) ||
!FLAGS_SET(c.inheritable, q->inheritable)) {
c.effective |= q->effective;
c.permitted |= q->permitted;
c.inheritable |= q->inheritable;
/* Now, let's enforce the caps for the first time. Note that this is where we acquire
* caps in any of the sets we currently don't have. We have to do this before
* dropping the bounding caps below, since at that point we can never acquire new
* caps in inherited/permitted/effective anymore, but only lose them.
*
* In order to change the bounding caps, we need to keep CAP_SETPCAP for a bit
* longer. Let's add it to our list hence for now. */
if (q->bounding != CAP_MASK_UNSET &&
(!BIT_SET(c.effective, CAP_SETPCAP) || !BIT_SET(c.permitted, CAP_SETPCAP))) {
CapabilityQuintet tmp = c;
SET_BIT(c.effective, CAP_SETPCAP);
SET_BIT(c.permitted, CAP_SETPCAP);
modified = true;
r = capability_apply(&tmp);
} else
r = capability_apply(&c);
if (r < 0)
return r;
}
}
if (q->bounding != CAP_MASK_UNSET) {
r = capability_bounding_set_drop(q->bounding, /* right_now= */ false);
if (r < 0)
return r;
}
/* If needed, let's now set the caps again, this time in the final version, which differs from what
* we have already set only in the CAP_SETPCAP bit, which we needed for dropping the bounding
* bits. This call only undoes bits and doesn't acquire any which means the bounding caps don't
* matter. */
if (modified) {
r = capability_apply(&c);
if (r < 0)
return r;
}
return 0;
}
int capability_get_ambient(uint64_t *ret) {
uint64_t a = 0;
int r;
assert(ret);
for (unsigned i = 0; i <= cap_last_cap(); i++) {
r = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, i, 0, 0);
if (r < 0)
return -errno;
if (r > 0)
SET_BIT(a, i);
}
*ret = a;
return 1;
}
int pidref_get_capability(const PidRef *pidref, CapabilityQuintet *ret) {
int r;
if (!pidref_is_set(pidref))
return -ESRCH;
if (pidref_is_remote(pidref))
return -EREMOTE;
const char *path = procfs_file_alloca(pidref->pid, "status");
_cleanup_fclose_ FILE *f = fopen(path, "re");
if (!f) {
if (errno == ENOENT && proc_mounted() == 0)
return -ENOSYS;
return -errno;
}
CapabilityQuintet q = CAPABILITY_QUINTET_NULL;
for (;;) {
_cleanup_free_ char *line = NULL;
r = read_line(f, LONG_LINE_MAX, &line);
if (r < 0)
return r;
if (r == 0)
break;
static const struct {
const char *field;
size_t offset;
} fields[] = {
{ "CapBnd:", offsetof(CapabilityQuintet, bounding) },
{ "CapInh:", offsetof(CapabilityQuintet, inheritable) },
{ "CapPrm:", offsetof(CapabilityQuintet, permitted) },
{ "CapEff:", offsetof(CapabilityQuintet, effective) },
{ "CapAmb:", offsetof(CapabilityQuintet, ambient) },
};
FOREACH_ELEMENT(i, fields) {
const char *p = first_word(line, i->field);
if (!p)
continue;
uint64_t *v = (uint64_t*) ((uint8_t*) &q + i->offset);
if (*v != CAP_MASK_UNSET)
return -EBADMSG;
r = safe_atoux64(p, v);
if (r < 0)
return r;
if (*v == CAP_MASK_UNSET)
return -EBADMSG;
}
}
if (!capability_quintet_is_fully_set(&q))
return -EBADMSG;
r = pidref_verify(pidref);
if (r < 0)
return r;
if (ret)
*ret = q;
return 0;
}
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