Subject: Re: [PATCH 5/7] submodule: handle NULL value when parsing submodule.*.branch

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 7 Dec 2023 09:14:48 +0100

On Thu, Dec 07, 2023 at 02:11:29AM -0500, Jeff King wrote:
> We record the submodule branch config value as a string, so config that
> uses an implicit bool like:
> 
>   [submodule "foo"]
>   branch
> 
> will cause us to segfault. Note that unlike most other config-parsing
> bugs of this class, this can be triggered by parsing a bogus .gitmodules
> file (which we might do after cloning a malicious repository).
> 
> I don't think the security implications are important, though. It's
> always a strict NULL dereference, not an out-of-bounds read or write. So
> we should reliably kill the process. That may be annoying, but the
> impact is limited to the attacker preventing the victim from
> successfully using "git clone --recurse-submodules", etc, on the
> malicious repo.
> 
> The "branch" entry is the only one with this problem; other strings like
> "path" and "url" already check for NULL.
> 
> Signed-off-by: Jeff King <[email-masked]>
> ---
>  submodule-config.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/submodule-config.c b/submodule-config.c
> index 6a48fd12f6..f4dd482abc 100644
> --- a/submodule-config.c
> +++ b/submodule-config.c
> @@ -516,7 +516,9 @@ static int parse_config(const char *var, const char *value,
>  			submodule->recommend_shallow =
>  				git_config_bool(var, value);
>  	} else if (!strcmp(item.buf, "branch")) {
> -		if (!me->overwrite && submodule->branch)
> +		if (!value)
> +			ret = config_error_nonbool(var);
> +		else if (!me->overwrite && submodule->branch)
>  			warn_multiple_config(me->treeish_name, submodule->name,
>  					     "branch");
>  		else {

I was about to say that I'd rather expect us to handle this gracefully
so that Git doesn't die when parsing an invalid gitmodules file. But
there are other cases where we already fail in the same way, so this
looks good to me.

Patrick