Subject: Re: [PATCH] http: redact curl h2h3 headers in info

From: Emily Shaffer <[email-masked]>
Date: Thu, 10 Nov 2022 13:57:35 -0800

On Wed, Nov 09, 2022 at 12:52:31AM +0000, Glen Choo via GitGitGadget wrote:
> 
> With GIT_TRACE_CURL=1 or GIT_CURL_VERBOSE=1, sensitive headers like
> "Authorization" and "Cookie" get redacted. However, since [1], curl's
> h2h3 module also prints headers in its "info", which don't get redacted.
> For example,
> 
>   echo 'github.com	TRUE	/	FALSE	1698960413304	o	foo=bar' >cookiefile &&
>   GIT_TRACE_CURL=1 GIT_TRACE_CURL_NO_DATA=1 git \
>     -c 'http.cookiefile=cookiefile' \
>     -c 'http.version=' \
>     ls-remote https://github.com/git/git refs/heads/main 2>output &&
>   grep 'cookie' output
> 
> produces output like:
> 
>   23:04:16.920495 http.c:678              == Info: h2h3 [cookie: o=foo=bar]
>   23:04:16.920562 http.c:637              => Send header: cookie: o=<redacted>
> 
> Teach http.c to check for h2h3 headers in info and redact them using the
> existing header redaction logic.
> 
> [1] https://github.com/curl/curl/commit/f8c3724aa90472c0e617ddbbc420aa199971eb77
> 
> Signed-off-by: Glen Choo <[email-masked]>
> ---
>     http: redact curl h2h3 headers in info
>     
>     I initially sent this to the security list, but the general impression
>     is that this isn't sensitive enough for an embargoed fix, so this is
>     better discussed in the open instead.
>     
>     Since this comes from curl's HTTP2.0/3.0 module, this can be mitigated
>     by setting http.version to 1.X, e.g. "git -c http.version=HTTP/1.1".
>     
>     According to [1], the susceptible curl versions appear to be 7.86.0,
>     7.85.0, 7.84.0, 7.83.1, 7.83.0, 7.82.0, but I'm not sure which platforms
>     are vulnerable.
>     
>     This patch fixes the issue on my machine running curl 7.85.0, so I think
>     it is okay to merge as-is. That said, I would strongly prefer to add
>     tests, but I haven't figured out how. In particular:
>     
>      * Do we have a way of using HTTP/2.0 in our tests? A cursory glance at
>        our httpd config suggests that we only use HTTP/1.1.
>      * How could we set up end-to-end tests to ensure that we're testing
>        this against affected versions of curl? To avoid regressions, I'd
>        also prefer to test against future versions of curl too.
>     
>     [1]
>     https://github.com/curl/curl/commit/f8c3724aa90472c0e617ddbbc420aa199971eb77
> 
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1377%2Fchooglen%2Fhttp%2Fredact-h2h3-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1377/chooglen/http/redact-h2h3-v1
> Pull-Request: https://github.com/git/git/pull/1377
> 
>  http.c | 40 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/http.c b/http.c
> index 5d0502f51fd..cbcc7c3f5b6 100644
> --- a/http.c
> +++ b/http.c
> @@ -560,8 +560,10 @@ static void set_curl_keepalive(CURL *c)
>  }
>  #endif
>  
> -static void redact_sensitive_header(struct strbuf *header)
> +/* Return 0 if redactions been made, 1 otherwise. */

Does it make sense to reverse the retval here?

`if (!redact_sensitive_header())` sounds like "if not redacted, ..." -
but here it means the opposite, right?

> +static int redact_sensitive_header(struct strbuf *header)
>  {
> +	int ret = 1;
>  	const char *sensitive_header;
>  
>  	if (trace_curl_redact &&
> @@ -575,6 +577,7 @@ static void redact_sensitive_header(struct strbuf *header)
>  		/* Everything else is opaque and possibly sensitive */
>  		strbuf_setlen(header,  sensitive_header - header->buf);
>  		strbuf_addstr(header, " <redacted>");
> +		ret = 0;
>  	} else if (trace_curl_redact &&
>  		   skip_iprefix(header->buf, "Cookie:", &sensitive_header)) {
>  		struct strbuf redacted_header = STRBUF_INIT;
> @@ -612,6 +615,27 @@ static void redact_sensitive_header(struct strbuf *header)
>  
>  		strbuf_setlen(header, sensitive_header - header->buf);
>  		strbuf_addbuf(header, &redacted_header);
> +		ret = 0;
> +	}
> +	return ret;
> +}
> +
> +/* Redact headers in info */
> +static void redact_sensitive_info_header(struct strbuf *header)
> +{
> +	const char *sensitive_header;
> +
> +	if (trace_curl_redact &&
> +	    skip_iprefix(header->buf, "h2h3 [", &sensitive_header)) {
> +		struct strbuf inner = STRBUF_INIT;
> +
> +		/* Drop the trailing "]" */
> +		strbuf_add(&inner, sensitive_header, strlen(sensitive_header) - 1);
> +		if (!redact_sensitive_header(&inner)) {
> +			strbuf_setlen(header, strlen("h2h3 ["));
> +			strbuf_addbuf(header, &inner);
> +			strbuf_addch(header, ']');

I'd really like some more comments in this function - even just one
describing the string we're trying to redact, or showing a sample line.
Navigating string parsing is always a bit difficult.

> +		}
>  	}
>  }
>  
> @@ -668,6 +692,18 @@ static void curl_dump_data(const char *text, unsigned char *ptr, size_t size)
>  	strbuf_release(&out);
>  }
>  
> +static void curl_print_info(char *data, size_t size)

Nit: Every other helper in this file calls it _dump_, so should this
also say _dump_ instead of _print_?

> +{
> +	struct strbuf buf = STRBUF_INIT;
> +
> +	strbuf_add(&buf, data, size);
> +
> +	redact_sensitive_info_header(&buf);
> +	trace_printf_key(&trace_curl, "== Info: %s", buf.buf);
> +
> +	strbuf_release(&buf);
> +}
> +
>  static int curl_trace(CURL *handle, curl_infotype type, char *data, size_t size, void *userp)
>  {
>  	const char *text;
> @@ -675,7 +711,7 @@ static int curl_trace(CURL *handle, curl_infotype type, char *data, size_t size,
>  
>  	switch (type) {
>  	case CURLINFO_TEXT:
> -		trace_printf_key(&trace_curl, "== Info: %s", data);
> +		curl_print_info(data, size);
>  		break;
>  	case CURLINFO_HEADER_OUT:
>  		text = "=> Send header";
> 
> base-commit: c03801e19cb8ab36e9c0d17ff3d5e0c3b0f24193

Otherwise functionally it seems fine to me. case CURLINFO_TEXT is the
one case that's not already using a curl_dump_* helper, so we're adding
one, and to that helper we're adding a call out to
redact_sensitive_header().

Thanks.
 - Emily

> -- 
> gitgitgadget