timesyncd: split privilege dropping code out of timesyncd so that we can make use of it from other daemons too

This is preparation to make networkd work as unpriviliged user.

3 files changed, 73 insertions, 3 deletions

diff --git a/src/shared/capability.c b/src/shared/capability.c
index b49c514009..69e054b1ff 100644
--- a/src/shared/capability.c
+++ b/src/shared/capability.c
@@ -29,12 +29,13 @@
 #include <ctype.h>
 #include <sys/capability.h>
 #include <sys/prctl.h>
+#include "grp.h"
 
 #include "macro.h"
-#include "capability.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;
@@ -212,3 +213,71 @@ int capability_bounding_set_drop_usermode(uint64_t drop) {
 
         return r;
 }
+
+int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilites) {
+
+        _cleanup_cap_free_ cap_t d = NULL;
+        cap_value_t bits[sizeof(keep_capabilites)*8];
+        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) {
+                log_error("Failed change group ID: %m");
+                return -errno;
+        }
+
+        if (setgroups(0, NULL) < 0) {
+                log_error("Failed to drop auxiliary groups list: %m");
+                return -errno;
+        }
+
+        if (prctl(PR_SET_KEEPCAPS, 1) < 0) {
+                log_error("Failed to enable keep capabilities flag: %m");
+                return -errno;
+        }
+
+        r = setresuid(uid, uid, uid);
+        if (r < 0) {
+                log_error("Failed change user ID: %m");
+                return -errno;
+        }
+
+        if (prctl(PR_SET_KEEPCAPS, 0) < 0) {
+                log_error("Failed to disable keep capabilities flag: %m");
+                return -errno;
+        }
+
+        r = capability_bounding_set_drop(~keep_capabilites, true);
+        if (r < 0) {
+                log_error("Failed to drop capabilities: %s", strerror(-r));
+                return r;
+        }
+
+        d = cap_init();
+        if (!d)
+                return log_oom();
+
+        for (i = 0; i < sizeof(keep_capabilites)*8; i++)
+                if (keep_capabilites & (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("Failed to enable capabilities bits: %m");
+                return -errno;
+        }
+
+        if (cap_set_proc(d) < 0) {
+                log_error("Failed to increase capabilities: %m");
+                return -errno;
+        }
+
+        return 0;
+}
diff --git a/src/shared/capability.h b/src/shared/capability.h
index 64f86410ab..3e6d9995f5 100644
--- a/src/shared/capability.h
+++ b/src/shared/capability.h
@@ -32,6 +32,8 @@ int have_effective_cap(int value);
 int capability_bounding_set_drop(uint64_t drop, bool right_now);
 int capability_bounding_set_drop_usermode(uint64_t drop);
 
+int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilites);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(cap_t, cap_free);
 #define _cleanup_cap_free_ _cleanup_(cap_freep)
 
diff --git a/src/shared/util.c b/src/shared/util.c
index 17b0ae16cf..4a3e35f356 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -6480,8 +6480,7 @@ void hexdump(FILE *f, const void *p, size_t s) {
         }
 }
 
-int update_reboot_param_file(const char *param)
-{
+int update_reboot_param_file(const char *param) {
         int r = 0;
 
         if (param) {