ask-password: refuse agent requests with unsafe characters in prompt fields

The message, icon and id fields are written verbatim into single-line
assignments of the [Ask] section of the agent request file, so a newline in
them lets the caller append arbitrary further assignments. Agents let a later
assignment override an earlier one, so an injected Socket= line redirects the
password to a path of the injector's choosing. Validate the fields and refuse
the request instead.

src/shared/ask-password-api.c

@@ -828,6 +828,21 @@ static int create_socket(const char *askpwdir, char **ret) {
         return TAKE_FD(fd);
 }
 
+bool ask_password_agent_prompt_fields_are_safe(const AskPasswordRequest *req) {
+        /* These fields end up in single-line "Key=value" assignments in the agent request file, hence a
+         * newline (or any other control character) in them would let the caller add further assignments to
+         * the [Ask] section, which agents then happily honour. Only the characters string_is_safe() rejects
+         * by default matter here, the rest is fine to appear in a prompt. */
+        const StringSafeFlags flags = STRING_ALLOW_EMPTY | STRING_ALLOW_BACKSLASHES |
+                                      STRING_ALLOW_QUOTES | STRING_ALLOW_GLOBS;
+
+        assert(req);
+
+        return (!req->message || string_is_safe(req->message, flags)) &&
+               (!req->icon || string_is_safe(req->icon, flags)) &&
+               (!req->id || string_is_safe(req->id, flags));
+}
+
 int ask_password_agent(
                 const AskPasswordRequest *req,
                 AskPasswordFlags flags,
@@ -850,6 +865,10 @@ int ask_password_agent(
         if (req->flag_file)
                 return -EOPNOTSUPP;
 
+        if (!ask_password_agent_prompt_fields_are_safe(req))
+                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Password request contains unsafe characters, refusing.");
+
         _cleanup_free_ char *askpwdir = NULL;
         r = get_ask_password_directory_for_flags(flags, &askpwdir);
         if (r < 0)

src/shared/ask-password-api.h

@@ -32,6 +32,7 @@ typedef struct AskPasswordRequest {
 
 int ask_password_tty(const AskPasswordRequest *req, AskPasswordFlags flags, char ***ret);
 int ask_password_plymouth(const AskPasswordRequest *req, AskPasswordFlags flags, char ***ret);
+bool ask_password_agent_prompt_fields_are_safe(const AskPasswordRequest *req) _pure_;
 int ask_password_agent(const AskPasswordRequest *req, AskPasswordFlags flags, char ***ret);
 int ask_password_auto(const AskPasswordRequest *req, AskPasswordFlags flags, char ***ret);