Correctly handle bogusly large chunk sizes

This fixes a denial of service attack vector where bogusly large chunk
sizes in requests could be used to force restarts of the Varnish
server.

This is Varnish Security Vulnerability VSV00001

For more information visit: https://varnish-cache.org/security/VSV00001

Fixes: #2379

bin/varnishd/http1/cache_http1_vfp.c

@@ -152,7 +152,7 @@ v1f_pull_chunked(struct vfp_ctx *vc, struct vfp_entry *vfe, void *ptr,
 		if (q == NULL || *q != '\0')
 			return (VFP_Error(vc, "chunked header number syntax"));
 		cl = (ssize_t)cll;
-		if ((uintmax_t)cl != cll)
+		if (cl < 0 || (uintmax_t)cl != cll)
 			return (VFP_Error(vc, "bogusly large chunk size"));
 
 		vfe->priv2 = cl;

bin/varnishtest/tests/f00001.vtc

@@ -0,0 +1,40 @@
+varnishtest "Check that we handle bogusly large chunks correctly"
+
+# Check that the bug has been fixed
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+} -start
+
+client c1 {
+	send "POST / HTTP/1.1\r\n"
+	send "Transfer-Encoding: chunked\r\n\r\n"
+	send "FFFFFFFFFFFFFFED\r\n"
+	send "0\r\n\r\n"
+
+	rxresp
+	expect resp.status == 503
+} -run
+
+# Check that the published workaround does not cause harm
+
+varnish v1 -vcl+backend {
+	sub vcl_recv {
+		if (req.http.transfer-encoding ~ "(?i)chunked") {
+			return (fail);
+		}
+	}
+}
+
+client c1 {
+	send "POST / HTTP/1.1\r\n"
+	send "Transfer-Encoding: chunked\r\n\r\n"
+	send "FFFFFFFFFFFFFFED\r\n"
+
+	rxresp
+	expect resp.status == 503
+} -run