BUG/MAJOR: h2: enforce stricter syntax checks on the :method pseudo-header

Before HTX was introduced, all the HTTP request elements passed in
pseudo-headers fields were used to build an HTTP/1 request whose syntax
was then scrutinized by the HTTP/1 parser, leaving no room to inject
invalid characters.

While NUL, CR and LF are properly blocked, it is possible to inject
spaces in the method so that once translated to HTTP/1, fields are
shifted by one spcae, and a lenient HTTP/1 server could possibly be
fooled into using a part of the method as the URI. For example, the
following request:

   H2 request
     :method: "GET /admin? HTTP/1.1"
     :path:   "/static/images"

would become:

   GET /admin? HTTP/1.1 /static/images HTTP/1.1

It's important to note that the resulting request is *not* valid, and
that in order for this to be a problem, it requires that this request
is delivered to an already vulnerable HTTP/1 server.

A workaround here is to reject malformed methods by placing this rule
in the frontend or backend, at least before leaving haproxy in H1:

   http-request reject if { method -m reg [^A-Z0-9] }

Alternately H2 may be globally disabled by commenting out the "alpn"
directive on "bind" lines, and by rejecting H2 streams creation by
adding the following statement to the global section:

   tune.h2.max-concurrent-streams 0

This patch adds a check for each character of the method to make sure
they belong to the ones permitted in a token, as mentioned in RFC7231#4.1.

This should be backported to versions 2.0 and above. For older versions
not having HTX_FL_PARSING_ERROR, a "goto fail" works as well as it
results in a protocol error at the stream level. Non-HTX versions are
safe because the resulting invalid request will be rejected by the
internal HTTP/1 parser.

Thanks to Tim D�sterhus for reporting that one.

src/h2.c

@@ -328,6 +328,14 @@ static struct htx_sl *h2_prepare_htx_reqline(uint32_t fields, struct ist *phdr,
 			flags |= HTX_SL_F_HAS_AUTHORITY;
 	}
 
+	/* The method is a non-empty token (RFC7231#4.1) */
+	if (!meth_sl.len)
+		goto fail;
+	for (i = 0; i < meth_sl.len; i++) {
+		if (!HTTP_IS_TOKEN(meth_sl.ptr[i]))
+			htx->flags |= HTX_FL_PARSING_ERROR;
+	}
+
 	/* make sure the final URI isn't empty. Note that 7540#8.1.2.3 states
 	 * that :path must not be empty.
 	 */