systemctl: unset const char* arguments in static destructors

When fuzzing, the following happens:
- we parse 'data' and produce an argv array,
- one of the items in argv is assigned to arg_host,
- the argv array is subsequently freed by strv_freep(), and arg_host has a dangling symlink.

In normal use, argv is static, so arg_host can never become a dangling pointer.
In fuzz-systemctl-parse-argv, if we repeatedly parse the same array, we
have some dangling pointers while we're in the middle of parsing. If we parse
the same array a second time, at the end all the dangling pointers will have been
replaced again. But for a short time, if parsing one of the arguments uses another
argument, we would use a dangling pointer.

Such a case occurs when we have --host=… --boot-loader-entry=help. The latter calls
acquire_bus() which uses arg_host.

I'm not particularly happy with making the code more complicated just for
fuzzing, but I think it's better to resolve this, even if the issue cannot
occur in normal invocations, than to deal with fuzzer reports.

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31714.

1 files changed, 10 insertions, 5 deletions

diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index a4100bd554..e37569ab7f 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -65,7 +65,7 @@ char **arg_states = NULL;
 char **arg_properties = NULL;
 bool arg_all = false;
 enum dependency arg_dependency = DEPENDENCY_FORWARD;
-const char *arg_job_mode = "replace";
+const char *_arg_job_mode = NULL;
 UnitFileScope arg_scope = UNIT_FILE_SYSTEM;
 bool arg_wait = false;
 bool arg_no_block = false;
@@ -115,8 +115,13 @@ bool arg_marked = false;
 STATIC_DESTRUCTOR_REGISTER(arg_types, strv_freep);
 STATIC_DESTRUCTOR_REGISTER(arg_states, strv_freep);
 STATIC_DESTRUCTOR_REGISTER(arg_properties, strv_freep);
+STATIC_DESTRUCTOR_REGISTER(_arg_job_mode, unsetp);
 STATIC_DESTRUCTOR_REGISTER(arg_wall, strv_freep);
+STATIC_DESTRUCTOR_REGISTER(arg_kill_who, unsetp);
 STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
+STATIC_DESTRUCTOR_REGISTER(arg_reboot_argument, unsetp);
+STATIC_DESTRUCTOR_REGISTER(arg_host, unsetp);
+STATIC_DESTRUCTOR_REGISTER(arg_boot_loader_entry, unsetp);
 STATIC_DESTRUCTOR_REGISTER(arg_clean_what, strv_freep);
 
 static int systemctl_help(void) {
@@ -598,19 +603,19 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_JOB_MODE:
-                        arg_job_mode = optarg;
+                        _arg_job_mode = optarg;
                         break;
 
                 case ARG_FAIL:
-                        arg_job_mode = "fail";
+                        _arg_job_mode = "fail";
                         break;
 
                 case ARG_IRREVERSIBLE:
-                        arg_job_mode = "replace-irreversibly";
+                        _arg_job_mode = "replace-irreversibly";
                         break;
 
                 case ARG_IGNORE_DEPENDENCIES:
-                        arg_job_mode = "ignore-dependencies";
+                        _arg_job_mode = "ignore-dependencies";
                         break;
 
                 case ARG_USER: