Add support for dropping capabilities from the bounding set.

Android daemons such as adbd need to drop capabilities from their
bounding sets (to prevent processes they launch from gaining privileges
through file capabilities), but not from their runtime
(permitted|inheritable|effective) sets. Add support for this and rename
some capability-related code to make things clearer.

While in there, fix a comment in the Android makefile.

Bug: 27274137
Change-Id: I7cab7e3302bb34cd7859b9621906391104bf6b4e

3 files changed, 65 insertions, 15 deletions

diff --git a/Android.mk b/Android.mk
index 2e54ec2..9be2907 100644
--- a/Android.mk
+++ b/Android.mk
@@ -162,7 +162,7 @@ LOCAL_SHARED_LIBRARIES := $(minijailCommonLibraries)
 include $(BUILD_NATIVE_TEST)
 
 
-# test_minijail executable for brillo_Minijail test.
+# libminijail_test executable for brillo_Minijail test.
 # =========================================================
 include $(CLEAR_VARS)
 LOCAL_MODULE := libminijail_test
diff --git a/libminijail.c b/libminijail.c
index 2164186..f916633 100644
--- a/libminijail.c
+++ b/libminijail.c
@@ -92,7 +92,8 @@ struct minijail {
 		int gid:1;
 		int usergroups:1;
 		int suppl_gids:1;
-		int caps:1;
+		int use_caps:1;
+		int capbset_drop:1;
 		int vfs:1;
 		int enter_vfs:1;
 		int pids:1;
@@ -121,6 +122,7 @@ struct minijail {
 	size_t suppl_gid_count;
 	gid_t *suppl_gid_list;
 	uint64_t caps;
+	uint64_t cap_bset;
 	pid_t initpid;
 	int mountns_fd;
 	int netns_fd;
@@ -342,11 +344,42 @@ void API minijail_log_seccomp_filter_failures(struct minijail *j)
 
 void API minijail_use_caps(struct minijail *j, uint64_t capmask)
 {
+	/*
+	 * 'minijail_use_caps' configures a runtime-capabilities-only
+	 * environment, including a bounding set matching the thread's runtime
+	 * (permitted|inheritable|effective) sets.
+	 * Therefore, it will override any existing bounding set configurations
+	 * since the latter would allow gaining extra runtime capabilities from
+	 * file capabilities.
+	 */
+	if (j->flags.capbset_drop) {
+		warn("overriding bounding set configuration");
+		j->cap_bset = 0;
+		j->flags.capbset_drop = 0;
+	}
 	j->caps = capmask;
-	j->flags.caps = 1;
+	j->flags.use_caps = 1;
 }
 
-void API minijail_reset_signal_mask(struct minijail* j) {
+void API minijail_capbset_drop(struct minijail *j, uint64_t capmask)
+{
+	if (j->flags.use_caps) {
+		/*
+		 * 'minijail_use_caps' will have already configured a capability
+		 * bounding set matching the (permitted|inheritable|effective)
+		 * sets. Abort if the user tries to configure a separate
+		 * bounding set. 'minijail_capbset_drop' and 'minijail_use_caps'
+		 * are mutually exclusive.
+		 */
+		die("runtime capabilities already configured, can't drop "
+		    "bounding set separately");
+	}
+	j->cap_bset = capmask;
+	j->flags.capbset_drop = 1;
+}
+
+void API minijail_reset_signal_mask(struct minijail *j)
+{
 	j->flags.reset_signal_mask = 1;
 }
 
@@ -1201,6 +1234,18 @@ static unsigned int get_last_valid_cap()
 	return last_valid_cap;
 }
 
+static void drop_capbset(uint64_t keep_mask, unsigned int last_valid_cap)
+{
+	const uint64_t one = 1;
+	unsigned int i;
+	for (i = 0; i < sizeof(keep_mask) * 8 && i <= last_valid_cap; ++i) {
+		if (keep_mask & (one << i))
+			continue;
+		if (prctl(PR_CAPBSET_DROP, i))
+			pdie("could not drop capability from bounding set");
+	}
+}
+
 void drop_caps(const struct minijail *j, unsigned int last_valid_cap)
 {
 	cap_t caps = cap_get_proc();
@@ -1236,12 +1281,7 @@ void drop_caps(const struct minijail *j, unsigned int last_valid_cap)
 	 * have been used above to raise a capability that wasn't already
 	 * present. This requires CAP_SETPCAP, so we raised/kept it above.
 	 */
-	for (i = 0; i < sizeof(j->caps) * 8 && i <= last_valid_cap; ++i) {
-		if (j->caps & (one << i))
-			continue;
-		if (prctl(PR_CAPBSET_DROP, i))
-			pdie("prctl(PR_CAPBSET_DROP)");
-	}
+	drop_capbset(j->caps, last_valid_cap);
 
 	/* If CAP_SETPCAP wasn't specifically requested, now we remove it. */
 	if ((j->caps & (one << CAP_SETPCAP)) == 0) {
@@ -1303,7 +1343,7 @@ void API minijail_enter(const struct minijail *j)
 	 * since /proc can be unmounted before drop_caps() is called.
 	 */
 	unsigned int last_valid_cap = 0;
-	if (j->flags.caps)
+	if (j->flags.capbset_drop || j->flags.use_caps)
 		last_valid_cap = get_last_valid_cap();
 
 	if (j->flags.pids)
@@ -1356,7 +1396,15 @@ void API minijail_enter(const struct minijail *j)
 	if (j->flags.remount_proc_ro && remount_proc_readonly(j))
 		pdie("remount");
 
-	if (j->flags.caps) {
+	/*
+	 * If we're only dropping capabilities from the bounding set, but not
+	 * from the thread's (permitted|inheritable|effective) sets, do it now.
+	 */
+	if (j->flags.capbset_drop) {
+		drop_capbset(j->cap_bset, last_valid_cap);
+	}
+
+	if (j->flags.use_caps) {
 		/*
 		 * POSIX capabilities are a bit tricky. If we drop our
 		 * capability to change uids, our attempt to use setuid()
@@ -1377,7 +1425,7 @@ void API minijail_enter(const struct minijail *j)
 	 */
 	if (j->flags.no_new_privs) {
 		drop_ugid(j);
-		if (j->flags.caps)
+		if (j->flags.use_caps)
 			drop_caps(j, last_valid_cap);
 
 		set_seccomp_filter(j);
@@ -1392,7 +1440,7 @@ void API minijail_enter(const struct minijail *j)
 		set_seccomp_filter(j);
 
 		drop_ugid(j);
-		if (j->flags.caps)
+		if (j->flags.use_caps)
 			drop_caps(j, last_valid_cap);
 	}
 
@@ -1635,7 +1683,7 @@ int minijail_run_internal(struct minijail *j, const char *filename,
 	}
 
 	if (!use_preload) {
-		if (j->flags.caps)
+		if (j->flags.use_caps)
 			die("capabilities are not supported without "
 			    "LD_PRELOAD");
 	}
diff --git a/libminijail.h b/libminijail.h
index b0ca61d..8bd8b39 100644
--- a/libminijail.h
+++ b/libminijail.h
@@ -52,7 +52,9 @@ void minijail_no_new_privs(struct minijail *j);
 void minijail_use_seccomp_filter(struct minijail *j);
 void minijail_parse_seccomp_filters(struct minijail *j, const char *path);
 void minijail_log_seccomp_filter_failures(struct minijail *j);
+/* 'minijail_use_caps' and 'minijail_capbset_drop' are mutually exclusive. */
 void minijail_use_caps(struct minijail *j, uint64_t capmask);
+void minijail_capbset_drop(struct minijail *j, uint64_t capmask);
 void minijail_reset_signal_mask(struct minijail *j);
 void minijail_namespace_vfs(struct minijail *j);
 void minijail_namespace_enter_vfs(struct minijail *j, const char *ns_path);