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

3 files changed, 74 insertions, 0 deletions

diff --git a/src/basic/capability-util.c b/src/basic/capability-util.c
index 881f0f671e..49c2d61afe 100644
--- a/src/basic/capability-util.c
+++ b/src/basic/capability-util.c
@@ -96,6 +96,61 @@ unsigned long cap_last_cap(void) {
         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;
diff --git a/src/basic/capability-util.h b/src/basic/capability-util.h
index f6a48b7916..be41475441 100644
--- a/src/basic/capability-util.h
+++ b/src/basic/capability-util.h
@@ -36,6 +36,9 @@ int have_effective_cap(int value);
 int capability_bounding_set_drop(uint64_t keep, bool right_now);
 int capability_bounding_set_drop_usermode(uint64_t keep);
 
+int capability_ambient_set_apply(uint64_t set, bool also_inherit);
+int capability_update_inherited_set(cap_t caps, uint64_t ambient_set);
+
 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities);
 
 int drop_capability(cap_value_t cv);
diff --git a/src/basic/missing.h b/src/basic/missing.h
index 880e724cb4..2d2785bead 100644
--- a/src/basic/missing.h
+++ b/src/basic/missing.h
@@ -1129,3 +1129,19 @@ static inline key_serial_t request_key(const char *type, const char *description
 #ifndef KEY_SPEC_USER_KEYRING
 #define KEY_SPEC_USER_KEYRING -4
 #endif
+
+#ifndef PR_CAP_AMBIENT
+#define PR_CAP_AMBIENT 47
+#endif
+
+#ifndef PR_CAP_AMBIENT_IS_SET
+#define PR_CAP_AMBIENT_IS_SET 1
+#endif
+
+#ifndef PR_CAP_AMBIENT_RAISE
+#define PR_CAP_AMBIENT_RAISE 2
+#endif
+
+#ifndef PR_CAP_AMBIENT_CLEAR_ALL
+#define PR_CAP_AMBIENT_CLEAR_ALL 4
+#endif