varlink: optionally, mark all incoming message's "parameters" field as sensitive

So far the varlink logic honoured the "sensitive" flag of output
messages. Let's add something similar for input messages. Since we don't
really know incoming messages, the flag simply controls whether the
"parmaeters" field of all incoming messages should be marked as
sensitive.

Then, turn this on in the credentials logic and in homed, since both
deal with credentials.

src/shared/varlink.c

@@ -184,6 +184,7 @@ struct Varlink {
         bool allow_fd_passing_output:1;
 
         bool output_buffer_sensitive:1; /* whether to erase the output buffer after writing it to the socket */
+        bool input_sensitive:1; /* Whether incoming messages might be sensitive */
 
         int af; /* address family if socket; AF_UNSPEC if not socket; negative if not known */
 
@@ -703,7 +704,7 @@ static void varlink_clear(Varlink *v) {
 
         varlink_clear_current(v);
 
-        v->input_buffer = mfree(v->input_buffer);
+        v->input_buffer = v->input_sensitive ? erase_and_free(v->input_buffer) : mfree(v->input_buffer);
         v->output_buffer = v->output_buffer_sensitive ? erase_and_free(v->output_buffer) : mfree(v->output_buffer);
 
         v->input_control_buffer = mfree(v->input_control_buffer);
@@ -1022,7 +1023,8 @@ static int varlink_read(Varlink *v) {
 }
 
 static int varlink_parse_message(Varlink *v) {
-        const char *e, *begin;
+        const char *e;
+        char *begin;
         size_t sz;
         int r;
 
@@ -1047,6 +1049,8 @@ static int varlink_parse_message(Varlink *v) {
         sz = e - begin + 1;
 
         r = json_parse(begin, 0, &v->current, NULL, NULL);
+        if (v->input_sensitive)
+                explicit_bzero_safe(begin, sz);
         if (r < 0) {
                 /* If we encounter a parse failure flush all data. We cannot possibly recover from this,
                  * hence drop all buffered data now. */
@@ -1054,6 +1058,13 @@ static int varlink_parse_message(Varlink *v) {
                 return varlink_log_errno(v, r, "Failed to parse JSON: %m");
         }
 
+        if (v->input_sensitive) {
+                /* Mark the parameters subfield as sensitive right-away, if that's requested */
+                JsonVariant *parameters = json_variant_by_key(v->current, "parameters");
+                if (parameters)
+                        json_variant_sensitive(parameters);
+        }
+
         v->input_buffer_size -= sz;
 
         if (v->input_buffer_size == 0)
@@ -3097,6 +3108,13 @@ int varlink_set_allow_fd_passing_output(Varlink *v, bool b) {
         return 0;
 }
 
+int varlink_set_input_sensitive(Varlink *v) {
+        assert_return(v, -EINVAL);
+
+        v->input_sensitive = true;
+        return 0;
+}
+
 int varlink_server_new(VarlinkServer **ret, VarlinkServerFlags flags) {
         _cleanup_(varlink_server_unrefp) VarlinkServer *s = NULL;
         int r;
@@ -3325,6 +3343,9 @@ static int connect_callback(sd_event_source *source, int fd, uint32_t revents, v
 
         TAKE_FD(cfd);
 
+        if (FLAGS_SET(ss->server->flags, VARLINK_SERVER_INPUT_SENSITIVE))
+                varlink_set_input_sensitive(v);
+
         if (ss->server->connect_callback) {
                 r = ss->server->connect_callback(ss->server, v, ss->server->userdata);
                 if (r < 0) {

src/shared/varlink.h

@@ -47,7 +47,8 @@ typedef enum VarlinkServerFlags {
         VARLINK_SERVER_MYSELF_ONLY      = 1 << 1, /* Only accessible by our own UID */
         VARLINK_SERVER_ACCOUNT_UID      = 1 << 2, /* Do per user accounting */
         VARLINK_SERVER_INHERIT_USERDATA = 1 << 3, /* Initialize Varlink connection userdata from VarlinkServer userdata */
-        _VARLINK_SERVER_FLAGS_ALL = (1 << 4) - 1,
+        VARLINK_SERVER_INPUT_SENSITIVE  = 1 << 4, /* Automatically mark al connection input as sensitive */
+        _VARLINK_SERVER_FLAGS_ALL = (1 << 5) - 1,
 } VarlinkServerFlags;
 
 typedef int (*VarlinkMethod)(Varlink *link, JsonVariant *parameters, VarlinkMethodFlags flags, void *userdata);
@@ -154,6 +155,9 @@ VarlinkServer* varlink_get_server(Varlink *v);
 
 int varlink_set_description(Varlink *v, const char *d);
 
+/* Automatically mark the parameters part of incoming messages as security sensitive */
+int varlink_set_input_sensitive(Varlink *v);
+
 /* Create a varlink server */
 int varlink_server_new(VarlinkServer **ret, VarlinkServerFlags flags);
 VarlinkServer *varlink_server_ref(VarlinkServer *s);