Re: Content-Security-Policy for man.cgi

[Ingo Schwarze <schwarze@usta.de>]

Hi Anthony,

Anthony J. Bentley wrote on Sun, Nov 10, 2019 at 01:59:21AM -0700:

> Modern browsers respect the Content-Security-Policy header,
> which restricts where dynamic resources like CSS and JavaScript
> can be specified in an HTML document.

I tried to read the standard https://www.w3.org/TR/CSP/
but miserably failed to understand anything because there
is so much indirection: "to do what <other standard> defines
in section 1.17.42.0 to the objects <yet another standard>
defines in section 4.3.2.1, use the methods described in
<some fourth standard> in section 36932451927, but only unless
the conditions explained in sections 666.0b and <aaah!>
apply."  And when you follow the pointers, you only find
more indirections to yet more places...  :-[

Do you know a place where that stuff is explained in a more
accessible reference-manual style?

https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP
turned out to be the other extreme: so imprecise and so much
handwaving that i didn't really understand it either.


> Since man.openbsd.org hosts manuals from many sources, and there's
> always danger of a bug in mandoc that allows dangerous HTML content
> through, a policy of "default-src 'none'; style-src 'self'" would be
> appropriate: this allows external stylesheets loaded from a URL on
> the same domain, but prohibits external links

You mean, prohibits embedding content (not sure whether that is the
correct term) from other sites?  Linking *to* other sites still
appears to be permitted...


> and inline CSS;

I do understand why inline CSS is bad style and can harm accessibility
and prevent formatting that is well adapted to the currently used
browsing device - but i fail to understand why a *security*
policy would worry about inline CSS.  Isn't "inline" by definition
the most secure source of content conceivable?


> scripts are not allowed at all. (mandoc(1) no longer generates
> inline styles at all, right?)

In a very small number of places, it still does:

mdoc_html.c, mdoc_it_pre(), LIST_tag, ROFFT_BODY:
	print_otag(h, TAG_DD, "s", "width", "auto");
	<dd style="width: auto">

mdoc_html.c, mdoc_fn_pre():
	print_otag(h, TAG_VAR, "cs", "Fa", "white-space", "nowrap");
	<var class=Fa" style="white-space: nowrap">

tbl_html.c, html_tblopen():
	h->tblt = print_otag(h, TAG_TABLE, "c?ss", "tbl", ...
	<tbl class="tbl" border=1
             style="border-style: solid; border-top-style: double">

tbl_html.c, print_tbl():
	print_otag(h, TAG_TR, "ss",
            "border-left-style", lborder,
            "border-bottom-style", bborder);
	<tr style="border-left-style: solid; border-bottom-style: double">

tbl_html.c, print_tbl():
	print_otag(h, TAG_TD, "??sss", ...
	<td colspan=3 rowspan=2 style="vertical-align: top; text-align;
            center; border-right-style: solid">


> Index: cgi.c
> ===================================================================
> RCS file: /cvs/src/usr.bin/mandoc/cgi.c,v
> retrieving revision 1.106
> diff -u -p -r1.106 cgi.c
> --- cgi.c	1 Oct 2019 17:54:04 -0000	1.106
> +++ cgi.c	10 Nov 2019 08:48:46 -0000
> @@ -336,6 +336,7 @@ resp_begin_http(int code, const char *ms
>  
>  	printf("Content-Type: text/html; charset=utf-8\r\n"
>  	     "Cache-Control: no-cache\r\n"
> +	     "Content-Security-Policy: default-src 'none'; style-src 'self';\r\n"

So would it have to be?:

	"Content-Security-Policy: default-src 'none'; "
	"style-src 'self' 'unsafe-inline'\r\n"

I think the semicolon in "'self';\r\n" isn't needed, right?

>  	     "Pragma: no-cache\r\n"
>  	     "\r\n");

By the way, could you check whether the CSP in

  https://mandoc.bsd.lv/cgi-bin/cvsweb/cvsweb.cgi?cvsroot=cvsweb#rev4.10

makes any sense?

Thanks,
  Ingo
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