Re: [PATCH] builtin/gc.c: fix use-after-free in maintenance_unregister()

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Nov 15 2022, Taylor Blau wrote:

> While trying to fix a move based on an uninitialized value (along with a
> declaration after the first statement), be0fd57228
> (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 be0fd57228. 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 <me@ttaylorr.com>
> ---
> I am disappointed in myself for finding this only after I pushed out a
> hotfix to 'next' and rebuild the downstream branches.
>
> This should be a minimal fix on top of Ævar's patch to get 'next'
> building again.

I'm also disappointed in myself, sorry. I *did* test it locally with
valgrind, but obviously fat-fingered it somehow and tested the wrong
version. Sorry!

>  builtin/gc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/builtin/gc.c b/builtin/gc.c
> index d87cf84041..38882a1e35 100644
> --- a/builtin/gc.c
> +++ b/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 } };

Just "{ 0 }" here instead? I see it may have been copied from some older
pre-image though, and they'll do the same in either case, so it's not
important...