Subject: Re: [PATCH 10/22] send-pack: fix leaking push cert nonce

From: Junio C Hamano <[email-masked]>
Date: Wed, 04 Sep 2024 15:08:00 -0700

Patrick Steinhardt <[email-masked]> writes:

> When retrieving the push cert nonce from the server, we first store the
> constant returned by `server_feature_value()` and then, if the nonce is
> valid, we duplicate the nonce memory to extend its lifetime. We never
> free the latter and thus cause a memory leak.

"to extend its lifetime" -> "to a NUL-terminated string, so that we
can pass it to generate_push_cert()".

What is going on in this code path is this.

The other side may send a nonce in the server capability.  We die if
that nonce is bogus.  Otherwise we make a xmemdupz() copy because we
need to pass the nonce to generate_push_cert() that expects nonce to
be a NUL terminated string (the original server capability is a long
concatenation of capabilities and we learn the <ptr, len> for the
nonce).  The function cryptographically signs the ref update request
we have, together with the nonce we got from the server, so that the
other side can validate that it is signed by us, and the nonce serves
as a protection against replay attacks.

> diff --git a/send-pack.c b/send-pack.c
> index b224ef9fc5e..c37f6ab3c07 100644
> --- a/send-pack.c
> +++ b/send-pack.c
> @@ -501,7 +501,7 @@ int send_pack(struct send_pack_args *args,
>  	unsigned cmds_sent = 0;
>  	int ret;
>  	struct async demux;
> -	const char *push_cert_nonce = NULL;
> +	char *push_cert_nonce = NULL;
>  	struct packet_reader reader;
>  	int use_bitmaps;

This is a change necessary to avoid having to cast the parameter to
free().

> @@ -550,10 +550,11 @@ int send_pack(struct send_pack_args *args,
>  
>  	if (args->push_cert != SEND_PACK_PUSH_CERT_NEVER) {
>  		size_t len;
> -		push_cert_nonce = server_feature_value("push-cert", &len);
> -		if (push_cert_nonce) {
> -			reject_invalid_nonce(push_cert_nonce, len);
> -			push_cert_nonce = xmemdupz(push_cert_nonce, len);
> +		const char *nonce = server_feature_value("push-cert", &len);
> +
> +		if (nonce) {
> +			reject_invalid_nonce(nonce, len);
> +			push_cert_nonce = xmemdupz(nonce, len);

And this hunk become needed because push_cert_nonce cannot receive
the return value from server_feature_value() without stripping
constness.

>  		} else if (args->push_cert == SEND_PACK_PUSH_CERT_ALWAYS) {
>  			die(_("the receiving end does not support --signed push"));
>  		} else if (args->push_cert == SEND_PACK_PUSH_CERT_IF_ASKED) {
> @@ -771,5 +772,6 @@ int send_pack(struct send_pack_args *args,
>  	oid_array_clear(&commons);
>  	strbuf_release(&req_buf);
>  	strbuf_release(&cap_buf);
> +	free(push_cert_nonce);

And this is my fault to forget freeing.  Thanks for spotting and fixing.

>  	return ret;
>  }