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-rw-r--r--src/basic/capability-util.c607
1 files changed, 607 insertions, 0 deletions
diff --git a/src/basic/capability-util.c b/src/basic/capability-util.c
new file mode 100644
index 0000000..c9c0b50
--- /dev/null
+++ b/src/basic/capability-util.c
@@ -0,0 +1,607 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#include "alloc-util.h"
+#include "capability-util.h"
+#include "cap-list.h"
+#include "fileio.h"
+#include "log.h"
+#include "macro.h"
+#include "missing_prctl.h"
+#include "missing_threads.h"
+#include "parse-util.h"
+#include "user-util.h"
+#include "util.h"
+
+int have_effective_cap(int value) {
+ _cleanup_cap_free_ cap_t cap = NULL;
+ cap_flag_value_t fv = CAP_CLEAR; /* To avoid false-positive use-of-uninitialized-value error reported
+ * by fuzzers. */
+
+ cap = cap_get_proc();
+ if (!cap)
+ return -errno;
+
+ if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
+ return -errno;
+
+ return fv == CAP_SET;
+}
+
+unsigned cap_last_cap(void) {
+ static thread_local unsigned 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) {
+
+ if (p > 63) /* 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). Let's hence protect
+ * ourselves against that and always cap at 63 for now. */
+ p = 63;
+
+ saved = p;
+ valid = true;
+ return p;
+ }
+ }
+
+ /* fall back to syscall-probing for pre linux-3.2 */
+ p = MIN((unsigned long) CAP_LAST_CAP, 63U);
+
+ 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 < 63; 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) {
+ /* Add capabilities in the set to the inherited caps, drops capabilities not in the set.
+ * Do not apply them yet. */
+
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ cap_flag_value_t flag = set & (UINT64_C(1) << i) ? CAP_SET : CAP_CLEAR;
+ cap_value_t v;
+
+ v = (cap_value_t) i;
+
+ if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, flag) < 0)
+ return -errno;
+ }
+
+ return 0;
+}
+
+int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
+ _cleanup_cap_free_ cap_t caps = NULL;
+ int r;
+
+ /* Remove capabilities requested in ambient set, but not in the bounding set */
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ if (set == 0)
+ break;
+
+ if (FLAGS_SET(set, (UINT64_C(1) << i)) && prctl(PR_CAPBSET_READ, i) != 1) {
+ log_debug("Ambient capability %s requested but missing from bounding set,"
+ " suppressing automatically.", capability_to_name(i));
+ set &= ~(UINT64_C(1) << i);
+ }
+ }
+
+ /* Add the capabilities to the ambient set (an possibly also the inheritable set) */
+
+ /* Check that we can use PR_CAP_AMBIENT or quit early. */
+ if (!ambient_capabilities_supported())
+ return (set & all_capabilities()) == 0 ?
+ 0 : -EOPNOTSUPP; /* if actually no ambient caps are to be set, be silent,
+ * otherwise fail recognizably */
+
+ if (also_inherit) {
+ 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 (unsigned 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;
+ } 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)
+ if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_LOWER, i, 0, 0) < 0)
+ return -errno;
+
+ }
+ }
+
+ return 0;
+}
+
+int capability_gain_cap_setpcap(cap_t *ret_before_caps) {
+ _cleanup_cap_free_ cap_t caps = NULL;
+ cap_flag_value_t fv;
+ caps = cap_get_proc();
+ if (!caps)
+ return -errno;
+
+ if (cap_get_flag(caps, 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(caps);
+ if (!temp_cap)
+ return -errno;
+
+ if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0)
+ return -errno;
+
+ if (cap_set_proc(temp_cap) < 0)
+ log_debug_errno(errno, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
+
+ /* 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. */
+ }
+ if (ret_before_caps)
+ /* Return the capabilities as they have been before setting CAP_SETPCAP */
+ *ret_before_caps = TAKE_PTR(caps);
+
+ return 0;
+}
+
+int capability_bounding_set_drop(uint64_t keep, bool right_now) {
+ _cleanup_cap_free_ cap_t before_cap = NULL, after_cap = NULL;
+ 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. */
+
+ r = capability_gain_cap_setpcap(&before_cap);
+ if (r < 0)
+ return r;
+
+ after_cap = cap_dup(before_cap);
+ if (!after_cap)
+ return -errno;
+
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ cap_value_t v;
+
+ if ((keep & (UINT64_C(1) << 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;
+ }
+ 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) {
+ /* If there are no actual changes anyway then let's ignore this error. */
+ if (cap_compare(before_cap, after_cap) != 0)
+ r = -errno;
+ }
+
+ 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_C(0xFFFFFFFF);
+ hi = (after >> 32) & UINT32_C(0xFFFFFFFF);
+
+ 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(0, 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, 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) {
+ cap_value_t bits[log2u64(keep_capabilities) + 1];
+ _cleanup_cap_free_ cap_t d = NULL;
+ unsigned i, j = 0;
+
+ d = cap_init();
+ if (!d)
+ return log_oom();
+
+ 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 & (UINT64_C(1) << (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;
+}
+
+bool ambient_capabilities_supported(void) {
+ static int cache = -1;
+
+ if (cache >= 0)
+ return cache;
+
+ /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
+ * available. */
+
+ cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
+ !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
+
+ return cache;
+}
+
+bool capability_quintet_mangle(CapabilityQuintet *q) {
+ uint64_t combined, drop = 0;
+ bool ambient_supported;
+
+ assert(q);
+
+ combined = q->effective | q->bounding | q->inheritable | q->permitted;
+
+ ambient_supported = q->ambient != UINT64_MAX;
+ if (ambient_supported)
+ combined |= q->ambient;
+
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ unsigned long bit = UINT64_C(1) << i;
+ if (!FLAGS_SET(combined, bit))
+ continue;
+
+ if (prctl(PR_CAPBSET_READ, i) > 0)
+ continue;
+
+ drop |= bit;
+
+ log_debug("Not in the current bounding set: %s", capability_to_name(i));
+ }
+
+ q->effective &= ~drop;
+ q->bounding &= ~drop;
+ q->inheritable &= ~drop;
+ q->permitted &= ~drop;
+
+ if (ambient_supported)
+ q->ambient &= ~drop;
+
+ return drop != 0; /* Let the caller know we changed something */
+}
+
+int capability_quintet_enforce(const CapabilityQuintet *q) {
+ _cleanup_cap_free_ cap_t c = NULL, modified = NULL;
+ int r;
+
+ if (q->ambient != UINT64_MAX) {
+ bool changed = false;
+
+ c = cap_get_proc();
+ if (!c)
+ return -errno;
+
+ /* In order to raise the ambient caps set we first need to raise the matching
+ * inheritable + permitted cap */
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ uint64_t m = UINT64_C(1) << i;
+ cap_value_t cv = (cap_value_t) i;
+ cap_flag_value_t old_value_inheritable, old_value_permitted;
+
+ if ((q->ambient & m) == 0)
+ continue;
+
+ if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value_inheritable) < 0)
+ return -errno;
+ if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value_permitted) < 0)
+ return -errno;
+
+ if (old_value_inheritable == CAP_SET && old_value_permitted == CAP_SET)
+ continue;
+
+ if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, CAP_SET) < 0)
+ return -errno;
+ if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
+ return -errno;
+
+ changed = true;
+ }
+
+ if (changed)
+ if (cap_set_proc(c) < 0)
+ return -errno;
+
+ r = capability_ambient_set_apply(q->ambient, false);
+ if (r < 0)
+ return r;
+ }
+
+ if (q->inheritable != UINT64_MAX || q->permitted != UINT64_MAX || q->effective != UINT64_MAX) {
+ bool changed = false;
+
+ if (!c) {
+ c = cap_get_proc();
+ if (!c)
+ return -errno;
+ }
+
+ for (unsigned i = 0; i <= cap_last_cap(); i++) {
+ uint64_t m = UINT64_C(1) << i;
+ cap_value_t cv = (cap_value_t) i;
+
+ if (q->inheritable != UINT64_MAX) {
+ cap_flag_value_t old_value, new_value;
+
+ if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value) < 0) {
+ if (errno == EINVAL) /* If the kernel knows more caps than this
+ * version of libcap, then this will return
+ * EINVAL. In that case, simply ignore it,
+ * pretend it doesn't exist. */
+ continue;
+
+ return -errno;
+ }
+
+ new_value = (q->inheritable & m) ? CAP_SET : CAP_CLEAR;
+
+ if (old_value != new_value) {
+ changed = true;
+
+ if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, new_value) < 0)
+ return -errno;
+ }
+ }
+
+ if (q->permitted != UINT64_MAX) {
+ cap_flag_value_t old_value, new_value;
+
+ if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value) < 0) {
+ if (errno == EINVAL)
+ continue;
+
+ return -errno;
+ }
+
+ new_value = (q->permitted & m) ? CAP_SET : CAP_CLEAR;
+
+ if (old_value != new_value) {
+ changed = true;
+
+ if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, new_value) < 0)
+ return -errno;
+ }
+ }
+
+ if (q->effective != UINT64_MAX) {
+ cap_flag_value_t old_value, new_value;
+
+ if (cap_get_flag(c, cv, CAP_EFFECTIVE, &old_value) < 0) {
+ if (errno == EINVAL)
+ continue;
+
+ return -errno;
+ }
+
+ new_value = (q->effective & m) ? CAP_SET : CAP_CLEAR;
+
+ if (old_value != new_value) {
+ changed = true;
+
+ if (cap_set_flag(c, CAP_EFFECTIVE, 1, &cv, new_value) < 0)
+ return -errno;
+ }
+ }
+ }
+
+ if (changed) {
+ /* 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 != UINT64_MAX) {
+ cap_value_t cv = CAP_SETPCAP;
+
+ modified = cap_dup(c);
+ if (!modified)
+ return -ENOMEM;
+
+ if (cap_set_flag(modified, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
+ return -errno;
+ if (cap_set_flag(modified, CAP_EFFECTIVE, 1, &cv, CAP_SET) < 0)
+ return -errno;
+
+ if (cap_compare(modified, c) == 0) {
+ /* No change? then drop this nonsense again */
+ cap_free(modified);
+ modified = NULL;
+ }
+ }
+
+ /* 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. */
+ if (cap_set_proc(modified ?: c) < 0)
+ return -errno;
+ }
+ }
+
+ if (q->bounding != UINT64_MAX) {
+ r = capability_bounding_set_drop(q->bounding, 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)
+ if (cap_set_proc(c) < 0)
+ return -errno;
+
+ return 0;
+}
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