sd-bus: add API to check if a client has privileges

This is a generalization of the vtable privilege check we already have,
but exported, and hence useful when preparing for a polkit change.

This will deal with the complexity that on dbus1 one cannot trust the
capability field we retrieve via the bus, since it is read via
/proc/$$/stat (and thus might be out-of-date) rather than directly from
the message (like on kdbus) or bus connection (as for uid creds on
dbus1).

Also, port over all code to this new API.

src/hostname/hostnamed.c

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <sys/utsname.h>
+#include <sys/capability.h>
 
 #include "util.h"
 #include "strv.h"
@@ -425,7 +426,7 @@ static int method_set_hostname(sd_bus *bus, sd_bus_message *m, void *userdata, s
         if (streq_ptr(name, c->data[PROP_HOSTNAME]))
                 return sd_bus_reply_method_return(m, NULL);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.hostname1.set-hostname", interactive, error, method_set_hostname, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN, "org.freedesktop.hostname1.set-hostname", interactive, error, method_set_hostname, c);
         if (r < 0)
                 return r;
         if (r == 0)
@@ -467,7 +468,7 @@ static int method_set_static_hostname(sd_bus *bus, sd_bus_message *m, void *user
         if (streq_ptr(name, c->data[PROP_STATIC_HOSTNAME]))
                 return sd_bus_reply_method_return(m, NULL);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.hostname1.set-static-hostname", interactive, error, method_set_static_hostname, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN, "org.freedesktop.hostname1.set-static-hostname", interactive, error, method_set_static_hostname, c);
         if (r < 0)
                 return r;
         if (r == 0)
@@ -532,9 +533,10 @@ static int set_machine_info(Context *c, sd_bus *bus, sd_bus_message *m, int prop
          * same time as the static one, use the same policy action for
          * both... */
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, prop == PROP_PRETTY_HOSTNAME ?
-                          "org.freedesktop.hostname1.set-static-hostname" :
-                          "org.freedesktop.hostname1.set-machine-info", interactive, error, cb, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN,
+                                    prop == PROP_PRETTY_HOSTNAME ?
+                                    "org.freedesktop.hostname1.set-static-hostname" :
+                                    "org.freedesktop.hostname1.set-machine-info", interactive, error, cb, c);
         if (r < 0)
                 return r;
         if (r == 0)

src/libsystemd/sd-bus/bus-convenience.c

@@ -472,3 +472,56 @@ _public_ int sd_bus_query_sender_creds(sd_bus_message *call, uint64_t mask, sd_b
 
         return bus_creds_extend_by_pid(c, mask, creds);
 }
+
+_public_ int sd_bus_query_sender_privilege(sd_bus_message *call, int capability) {
+        _cleanup_bus_creds_unref_ sd_bus_creds *creds = NULL;
+        uid_t our_uid;
+        int r;
+
+        assert_return(call, -EINVAL);
+        assert_return(call->sealed, -EPERM);
+        assert_return(call->bus, -EINVAL);
+        assert_return(!bus_pid_changed(call->bus), -ECHILD);
+
+        if (!BUS_IS_OPEN(call->bus->state))
+                return -ENOTCONN;
+
+        /* We only trust the effective capability set if this is
+         * kdbus. On classic dbus1 we cannot retrieve the value
+         * without races. Since this function is supposed to be useful
+         * for authentication decision we hence avoid requesting and
+         * using that information. */
+        if (call->bus->is_kernel && capability >= 0) {
+                r = sd_bus_query_sender_creds(call, SD_BUS_CREDS_UID|SD_BUS_CREDS_EFFECTIVE_CAPS, &creds);
+                if (r < 0)
+                        return r;
+
+                r = sd_bus_creds_has_effective_cap(creds, capability);
+                if (r > 0)
+                        return 1;
+        } else {
+                r = sd_bus_query_sender_creds(call, SD_BUS_CREDS_UID, &creds);
+                if (r < 0)
+                        return r;
+        }
+
+        /* Now, check the UID, but only if the capability check wasn't
+         * sufficient */
+        our_uid = getuid();
+        if (our_uid != 0 || !call->bus->is_kernel || capability < 0) {
+                uid_t sender_uid;
+
+                r = sd_bus_creds_get_uid(creds, &sender_uid);
+                if (r >= 0) {
+                        /* Sender has same UID as us, then let's grant access */
+                        if (sender_uid == our_uid)
+                                return 1;
+
+                        /* Sender is root, we are not root. */
+                        if (our_uid != 0 && sender_uid == 0)
+                                return 1;
+                }
+        }
+
+        return 0;
+}

src/libsystemd/sd-bus/bus-objects.c

@@ -289,7 +289,6 @@ static int node_callbacks_run(
 static int check_access(sd_bus *bus, sd_bus_message *m, struct vtable_member *c, sd_bus_error *error) {
         _cleanup_bus_creds_unref_ sd_bus_creds *creds = NULL;
         uint64_t cap;
-        uid_t uid;
         int r;
 
         assert(bus);
@@ -304,17 +303,6 @@ static int check_access(sd_bus *bus, sd_bus_message *m, struct vtable_member *c,
         if (c->vtable->flags & SD_BUS_VTABLE_UNPRIVILEGED)
                 return 0;
 
-        /* If we are not connected to kdbus we cannot retrieve the
-         * effective capability set without race. Since we need this
-         * for a security decision we cannot use racy data, hence
-         * don't request it. */
-        if (bus->is_kernel)
-                r = sd_bus_query_sender_creds(m, SD_BUS_CREDS_UID|SD_BUS_CREDS_EFFECTIVE_CAPS, &creds);
-        else
-                r = sd_bus_query_sender_creds(m, SD_BUS_CREDS_UID, &creds);
-        if (r < 0)
-                return r;
-
         /* Check have the caller has the requested capability
          * set. Note that the flags value contains the capability
          * number plus one, which we need to subtract here. We do this
@@ -328,16 +316,11 @@ static int check_access(sd_bus *bus, sd_bus_message *m, struct vtable_member *c,
         else
                 cap --;
 
-        r = sd_bus_creds_has_effective_cap(creds, cap);
+        r = sd_bus_query_sender_privilege(m, cap);
+        if (r < 0)
+                return r;
         if (r > 0)
-                return 1;
-
-        /* Caller has same UID as us, then let's grant access */
-        r = sd_bus_creds_get_uid(creds, &uid);
-        if (r >= 0) {
-                if (uid == getuid())
-                        return 1;
-        }
+                return 0;
 
         return sd_bus_error_setf(error, SD_BUS_ERROR_ACCESS_DENIED, "Access to %s.%s() not permitted.", c->interface, c->member);
 }

src/libsystemd/sd-bus/bus-util.c

@@ -186,28 +186,22 @@ int bus_name_has_owner(sd_bus *c, const char *name, sd_bus_error *error) {
 int bus_verify_polkit(
                 sd_bus *bus,
                 sd_bus_message *m,
+                int capability,
                 const char *action,
                 bool interactive,
                 bool *_challenge,
                 sd_bus_error *e) {
 
-        _cleanup_bus_creds_unref_ sd_bus_creds *creds = NULL;
-        uid_t uid;
         int r;
 
         assert(bus);
         assert(m);
         assert(action);
 
-        r = sd_bus_query_sender_creds(m, SD_BUS_CREDS_UID, &creds);
+        r = sd_bus_query_sender_privilege(m, capability);
         if (r < 0)
                 return r;
-
-        r = sd_bus_creds_get_uid(creds, &uid);
-        if (r < 0)
-                return r;
-
-        if (uid == 0)
+        if (r > 0)
                 return 1;
 
 #ifdef ENABLE_POLKIT
@@ -325,6 +319,7 @@ int bus_verify_polkit_async(
                 sd_bus *bus,
                 Hashmap **registry,
                 sd_bus_message *m,
+                int capability,
                 const char *action,
                 bool interactive,
                 sd_bus_error *error,
@@ -336,8 +331,6 @@ int bus_verify_polkit_async(
         AsyncPolkitQuery *q;
         const char *sender;
 #endif
-        _cleanup_bus_creds_unref_ sd_bus_creds *creds = NULL;
-        uid_t uid;
         int r;
 
         assert(bus);
@@ -383,15 +376,10 @@ int bus_verify_polkit_async(
         }
 #endif
 
-        r = sd_bus_query_sender_creds(m, SD_BUS_CREDS_UID, &creds);
+        r = sd_bus_query_sender_privilege(m, capability);
         if (r < 0)
                 return r;
-
-        r = sd_bus_creds_get_uid(creds, &uid);
-        if (r < 0)
-                return r;
-
-        if (uid == 0)
+        if (r > 0)
                 return 1;
 
 #ifdef ENABLE_POLKIT

src/libsystemd/sd-bus/bus-util.h

@@ -62,9 +62,9 @@ int bus_name_has_owner(sd_bus *c, const char *name, sd_bus_error *error);
 
 int bus_check_peercred(sd_bus *c);
 
-int bus_verify_polkit(sd_bus *bus, sd_bus_message *m, const char *action, bool interactive, bool *_challenge, sd_bus_error *e);
+int bus_verify_polkit(sd_bus *bus, sd_bus_message *m, int capability, const char *action, bool interactive, bool *_challenge, sd_bus_error *e);
 
-int bus_verify_polkit_async(sd_bus *bus, Hashmap **registry, sd_bus_message *m, const char *action, bool interactive, sd_bus_error *error, sd_bus_message_handler_t callback, void *userdata);
+int bus_verify_polkit_async(sd_bus *bus, Hashmap **registry, sd_bus_message *m, int capability, const char *action, bool interactive, sd_bus_error *error, sd_bus_message_handler_t callback, void *userdata);
 void bus_verify_polkit_async_registry_free(sd_bus *bus, Hashmap *registry);
 
 int bus_open_system_systemd(sd_bus **_bus);

src/locale/localed.c

@@ -23,6 +23,7 @@
 #include <errno.h>
 #include <string.h>
 #include <unistd.h>
+#include <sys/capability.h>
 
 #include "sd-bus.h"
 
@@ -876,7 +877,7 @@ static int method_set_locale(sd_bus *bus, sd_bus_message *m, void *userdata, sd_
         }
 
         if (modified) {
-                r = bus_verify_polkit_async(bus, &c->polkit_registry, m,
+                r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN,
                                             "org.freedesktop.locale1.set-locale", interactive,
                                             error, method_set_locale, c);
                 if (r < 0)
@@ -954,7 +955,7 @@ static int method_set_vc_keyboard(sd_bus *bus, sd_bus_message *m, void *userdata
                     (keymap_toggle && (!filename_is_safe(keymap_toggle) || !string_is_safe(keymap_toggle))))
                         return sd_bus_error_set_errnof(error, -EINVAL, "Received invalid keymap data");
 
-                r = bus_verify_polkit_async(bus, &c->polkit_registry, m,
+                r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN,
                                 "org.freedesktop.locale1.set-keyboard",
                                 interactive, error, method_set_vc_keyboard, c);
                 if (r < 0)
@@ -1026,7 +1027,7 @@ static int method_set_x11_keyboard(sd_bus *bus, sd_bus_message *m, void *userdat
                     (options && !string_is_safe(options)))
                         return sd_bus_error_set_errnof(error, -EINVAL, "Received invalid keyboard data");
 
-                r = bus_verify_polkit_async(bus, &c->polkit_registry, m,
+                r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_ADMIN,
                                 "org.freedesktop.locale1.set-keyboard",
                                 interactive, error, method_set_x11_keyboard, c);
                 if (r < 0)

src/login/logind-dbus.c

@@ -1032,6 +1032,7 @@ static int method_set_user_linger(sd_bus *bus, sd_bus_message *message, void *us
         r = bus_verify_polkit_async(bus,
                                     &m->polkit_registry,
                                     message,
+                                    CAP_SYS_ADMIN,
                                     "org.freedesktop.login1.set-user-linger",
                                     interactive,
                                     error,
@@ -1204,6 +1205,7 @@ static int method_attach_device(sd_bus *bus, sd_bus_message *message, void *user
         r = bus_verify_polkit_async(bus,
                                     &m->polkit_registry,
                                     message,
+                                    CAP_SYS_ADMIN,
                                     "org.freedesktop.login1.attach-device",
                                     interactive,
                                     error,
@@ -1235,6 +1237,7 @@ static int method_flush_devices(sd_bus *bus, sd_bus_message *message, void *user
         r = bus_verify_polkit_async(bus,
                                     &m->polkit_registry,
                                     message,
+                                    CAP_SYS_ADMIN,
                                     "org.freedesktop.login1.flush-devices",
                                     interactive,
                                     error,
@@ -1532,7 +1535,7 @@ static int method_do_shutdown_or_sleep(
         blocked = manager_is_inhibited(m, w, INHIBIT_BLOCK, NULL, false, true, uid, NULL);
 
         if (multiple_sessions) {
-                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message,
+                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message, CAP_SYS_BOOT,
                                             action_multiple_sessions, interactive, error, method, m);
                 if (r < 0)
                         return r;
@@ -1541,7 +1544,7 @@ static int method_do_shutdown_or_sleep(
         }
 
         if (blocked) {
-                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message,
+                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message, CAP_SYS_BOOT,
                                             action_ignore_inhibit, interactive, error, method, m);
                 if (r < 0)
                         return r;
@@ -1550,7 +1553,7 @@ static int method_do_shutdown_or_sleep(
         }
 
         if (!multiple_sessions && !blocked) {
-                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message,
+                r = bus_verify_polkit_async(m->bus, &m->polkit_registry, message, CAP_SYS_BOOT,
                                             action, interactive, error, method, m);
                 if (r < 0)
                         return r;
@@ -1688,7 +1691,7 @@ static int method_can_shutdown_or_sleep(
         blocked = manager_is_inhibited(m, w, INHIBIT_BLOCK, NULL, false, true, uid, NULL);
 
         if (multiple_sessions) {
-                r = bus_verify_polkit(m->bus, message, action_multiple_sessions, false, &challenge, error);
+                r = bus_verify_polkit(m->bus, message, CAP_SYS_BOOT, action_multiple_sessions, false, &challenge, error);
                 if (r < 0)
                         return r;
 
@@ -1701,7 +1704,7 @@ static int method_can_shutdown_or_sleep(
         }
 
         if (blocked) {
-                r = bus_verify_polkit(m->bus, message, action_ignore_inhibit, false, &challenge, error);
+                r = bus_verify_polkit(m->bus, message, CAP_SYS_BOOT, action_ignore_inhibit, false, &challenge, error);
                 if (r < 0)
                         return r;
 
@@ -1717,7 +1720,7 @@ static int method_can_shutdown_or_sleep(
                 /* If neither inhibit nor multiple sessions
                  * apply then just check the normal policy */
 
-                r = bus_verify_polkit(m->bus, message, action, false, &challenge, error);
+                r = bus_verify_polkit(m->bus, message, CAP_SYS_BOOT, action, false, &challenge, error);
                 if (r < 0)
                         return r;
 
@@ -1837,7 +1840,7 @@ static int method_inhibit(sd_bus *bus, sd_bus_message *message, void *userdata,
         if (m->action_what & w)
                 return sd_bus_error_setf(error, BUS_ERROR_OPERATION_IN_PROGRESS, "The operation inhibition has been requested for is already running");
 
-        r = bus_verify_polkit_async(bus, &m->polkit_registry, message,
+        r = bus_verify_polkit_async(bus, &m->polkit_registry, message, CAP_SYS_BOOT,
                                     w == INHIBIT_SHUTDOWN             ? (mm == INHIBIT_BLOCK ? "org.freedesktop.login1.inhibit-block-shutdown" : "org.freedesktop.login1.inhibit-delay-shutdown") :
                                     w == INHIBIT_SLEEP                ? (mm == INHIBIT_BLOCK ? "org.freedesktop.login1.inhibit-block-sleep"    : "org.freedesktop.login1.inhibit-delay-sleep") :
                                     w == INHIBIT_IDLE                 ? "org.freedesktop.login1.inhibit-block-idle" :

src/systemd/sd-bus.h

@@ -282,6 +282,7 @@ int sd_bus_emit_interfaces_removed_strv(sd_bus *bus, const char *path, char **in
 int sd_bus_emit_interfaces_removed(sd_bus *bus, const char *path, const char *interface, ...) _sd_sentinel_;
 
 int sd_bus_query_sender_creds(sd_bus_message *call, uint64_t mask, sd_bus_creds **creds);
+int sd_bus_query_sender_privilege(sd_bus_message *call, int capability);
 
 /* Credential handling */
 

src/timedate/timedated.c

@@ -395,7 +395,7 @@ static int method_set_timezone(sd_bus *bus, sd_bus_message *m, void *userdata, s
         if (streq_ptr(z, c->zone))
                 return sd_bus_reply_method_return(m, NULL);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.timedate1.set-timezone", interactive, error, method_set_timezone, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_TIME, "org.freedesktop.timedate1.set-timezone", interactive, error, method_set_timezone, c);
         if (r < 0)
                 return r;
         if (r == 0)
@@ -456,7 +456,7 @@ static int method_set_local_rtc(sd_bus *bus, sd_bus_message *m, void *userdata,
         if (lrtc == c->local_rtc)
                 return sd_bus_reply_method_return(m, NULL);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.timedate1.set-local-rtc", interactive, error, method_set_local_rtc, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_TIME, "org.freedesktop.timedate1.set-local-rtc", interactive, error, method_set_local_rtc, c);
         if (r < 0)
                 return r;
         if (r == 0)
@@ -561,7 +561,7 @@ static int method_set_time(sd_bus *bus, sd_bus_message *m, void *userdata, sd_bu
         } else
                 timespec_store(&ts, (usec_t) utc);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.timedate1.set-time", interactive, error, method_set_time, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_TIME, "org.freedesktop.timedate1.set-time", interactive, error, method_set_time, c);
         if (r < 0)
                 return r;
         if (r == 0)
@@ -601,7 +601,7 @@ static int method_set_ntp(sd_bus *bus, sd_bus_message *m, void *userdata, sd_bus
         if ((bool)ntp == c->use_ntp)
                 return sd_bus_reply_method_return(m, NULL);
 
-        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, "org.freedesktop.timedate1.set-ntp", interactive, error, method_set_ntp, c);
+        r = bus_verify_polkit_async(bus, &c->polkit_registry, m, CAP_SYS_TIME, "org.freedesktop.timedate1.set-ntp", interactive, error, method_set_ntp, c);
         if (r < 0)
                 return r;
         if (r == 0)