From b7a85e0003cedb1b48a1fd3ae5b746ec6330102e Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Wed, 7 Jul 2021 16:34:34 -0700
Subject: [PATCH] net/http/httputil: close incoming ReverseProxy request body

Reading from an incoming request body after the request handler aborts
with a panic can cause a panic, becuse http.Server does not (contrary
to its documentation) close the request body in this case.

Always close the incoming request body in ReverseProxy.ServeHTTP to
ensure that any in-flight outgoing requests using the body do not
read from it.

Updates #46866
Fixes CVE-2021-36221

Change-Id: I310df269200ad8732c5d9f1a2b00de68725831df
Reviewed-on: https://go-review.googlesource.com/c/go/+/333191
Trust: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
---
 src/net/http/httputil/reverseproxy.go      |  9 +++++
 src/net/http/httputil/reverseproxy_test.go | 39 ++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index 5d39955d62..8b63368386 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -235,6 +235,15 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	if req.ContentLength == 0 {
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
+	if outreq.Body != nil {
+		// Reading from the request body after returning from a handler is not
+		// allowed, and the RoundTrip goroutine that reads the Body can outlive
+		// this handler. This can lead to a crash if the handler panics (see
+		// Issue 46866). Although calling Close doesn't guarantee there isn't
+		// any Read in flight after the handle returns, in practice it's safe to
+		// read after closing it.
+		defer outreq.Body.Close()
+	}
 	if outreq.Header == nil {
 		outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
 	}
-- 
2.27.0

