builtin/gc.c: fix use-after-free in maintenance_unregister()

While trying to fix a move based on an uninitialized value (along with a
declaration after the first statement), be0fd57
(maintenance --unregister: fix uninit'd data use &
-Wdeclaration-after-statement, 2022-11-15) unintentionally introduced a
use-after-free.

The problem arises when `maintenance_unregister()` sees a non-NULL
`config_file` string and thus tries to call
git_configset_get_value_multi() to lookup the corresponding values.

We store the result off, and then call git_configset_clear(), which
frees the pointer that we just stored. We then try to read that
now-freed pointer a few lines below, and there we have our
use-after-free:

    $ ./t7900-maintenance.sh -vxi --run=23 --valgrind
    [...]
    + git maintenance unregister --config-file ./other
    ==3048727== Invalid read of size 8
    ==3048727==    at 0x1869CA: maintenance_unregister (gc.c:1590)
    ==3048727==    by 0x188F42: cmd_maintenance (gc.c:2651)
    ==3048727==    by 0x128C62: run_builtin (git.c:466)
    ==3048727==    by 0x12907E: handle_builtin (git.c:721)
    ==3048727==    by 0x1292EC: run_argv (git.c:788)
    ==3048727==    by 0x12988E: cmd_main (git.c:926)
    ==3048727==    by 0x21ED39: main (common-main.c:57)
    ==3048727==  Address 0x4b38bc8 is 24 bytes inside a block of size 64 free'd
    ==3048727==    at 0x484617B: free (vg_replace_malloc.c:872)
    ==3048727==    by 0x2D207E: free_individual_entries (hashmap.c:188)
    ==3048727==    by 0x2D2153: hashmap_clear_ (hashmap.c:207)
    ==3048727==    by 0x270B5C: git_configset_clear (config.c:2375)
    ==3048727==    by 0x1869AC: maintenance_unregister (gc.c:1585)
    ==3048727==    by 0x188F42: cmd_maintenance (gc.c:2651)
    ==3048727==    by 0x128C62: run_builtin (git.c:466)
    ==3048727==    by 0x12907E: handle_builtin (git.c:721)
    ==3048727==    by 0x1292EC: run_argv (git.c:788)
    ==3048727==    by 0x12988E: cmd_main (git.c:926)
    ==3048727==    by 0x21ED39: main (common-main.c:57)
    [...]

Resolve this via a partial-revert of be0fd57. The config_set struct
now gets a zero initialization, which makes free()-ing it a noop even
without calling git_configset_init(). When we do initialize it to a
non-zero value, it is only free()'d after our last read of `list`.

Signed-off-by: Taylor Blau <[email protected]>

Author: ttaylorr

builtin/gc.c

@@ -1543,6 +1543,7 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
 	int found = 0;
 	struct string_list_item *item;
 	const struct string_list *list;
+	struct config_set cs = { { 0 } };
 
 	argc = parse_options(argc, argv, prefix, options,
 			     builtin_maintenance_unregister_usage, 0);
@@ -1551,12 +1552,9 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
 				   options);
 
 	if (config_file) {
-		struct config_set cs;
-
 		git_configset_init(&cs);
 		git_configset_add_file(&cs, config_file);
 		list = git_configset_get_value_multi(&cs, key);
-		git_configset_clear(&cs);
 	} else {
 		list = git_config_get_value_multi(key);
 	}
@@ -1592,6 +1590,7 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
 		die(_("repository '%s' is not registered"), maintpath);
 	}
 
+	git_configset_clear(&cs);
 	free(maintpath);
 	return 0;
 }