--- /dev/null
+From fa98f46741f818913a8c11b877520a548715131f Mon Sep 17 00:00:00 2001
+From: Russ Cox <rsc@golang.org>
+Date: Mon, 13 Jul 2020 13:27:22 -0400
+Subject: [PATCH] net/http: synchronize "100 Continue" write and Handler writes
+
+The expectContinueReader writes to the connection on the first
+Request.Body read. Since a Handler might be doing a read in parallel or
+before a write, expectContinueReader needs to synchronize with the
+ResponseWriter, and abort if a response already went out.
+
+The tests will land in a separate CL.
+
+Fixes #34902
+Fixes CVE-2020-15586
+
+Change-Id: Icdd8dd539f45e8863762bd378194bb4741e875fc
+Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/793350
+Reviewed-by: Filippo Valsorda <valsorda@google.com>
+Reviewed-on: https://go-review.googlesource.com/c/go/+/242598
+Run-TryBot: Katie Hockman <katie@golang.org>
+Reviewed-by: Filippo Valsorda <filippo@golang.org>
+TryBot-Result: Gobot Gobot <gobot@golang.org>
+
+Upstream-Status: Backport
+CVE: CVE-2020-15586
+Signed-off-by: Li Zhou <li.zhou@windriver.com>
+---
+ src/net/http/server.go | 43 +++++++++++++++++++++++++++++++++++-------
+ 1 file changed, 36 insertions(+), 7 deletions(-)
+
+diff --git a/src/net/http/server.go b/src/net/http/server.go
+index a995a50658..d41b5f6f48 100644
+--- a/src/net/http/server.go
++++ b/src/net/http/server.go
+@@ -425,6 +425,16 @@ type response struct {
+ wants10KeepAlive bool // HTTP/1.0 w/ Connection "keep-alive"
+ wantsClose bool // HTTP request has Connection "close"
+
++ // canWriteContinue is a boolean value accessed as an atomic int32
++ // that says whether or not a 100 Continue header can be written
++ // to the connection.
++ // writeContinueMu must be held while writing the header.
++ // These two fields together synchronize the body reader
++ // (the expectContinueReader, which wants to write 100 Continue)
++ // against the main writer.
++ canWriteContinue atomicBool
++ writeContinueMu sync.Mutex
++
+ w *bufio.Writer // buffers output in chunks to chunkWriter
+ cw chunkWriter
+
+@@ -515,6 +525,7 @@ type atomicBool int32
+
+ func (b *atomicBool) isSet() bool { return atomic.LoadInt32((*int32)(b)) != 0 }
+ func (b *atomicBool) setTrue() { atomic.StoreInt32((*int32)(b), 1) }
++func (b *atomicBool) setFalse() { atomic.StoreInt32((*int32)(b), 0) }
+
+ // declareTrailer is called for each Trailer header when the
+ // response header is written. It notes that a header will need to be
+@@ -878,21 +889,27 @@ type expectContinueReader struct {
+ resp *response
+ readCloser io.ReadCloser
+ closed bool
+- sawEOF bool
++ sawEOF atomicBool
+ }
+
+ func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
+ if ecr.closed {
+ return 0, ErrBodyReadAfterClose
+ }
+- if !ecr.resp.wroteContinue && !ecr.resp.conn.hijacked() {
+- ecr.resp.wroteContinue = true
+- ecr.resp.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
+- ecr.resp.conn.bufw.Flush()
++ w := ecr.resp
++ if !w.wroteContinue && w.canWriteContinue.isSet() && !w.conn.hijacked() {
++ w.wroteContinue = true
++ w.writeContinueMu.Lock()
++ if w.canWriteContinue.isSet() {
++ w.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
++ w.conn.bufw.Flush()
++ w.canWriteContinue.setFalse()
++ }
++ w.writeContinueMu.Unlock()
+ }
+ n, err = ecr.readCloser.Read(p)
+ if err == io.EOF {
+- ecr.sawEOF = true
++ ecr.sawEOF.setTrue()
+ }
+ return
+ }
+@@ -1311,7 +1328,7 @@ func (cw *chunkWriter) writeHeader(p []byte) {
+ // because we don't know if the next bytes on the wire will be
+ // the body-following-the-timer or the subsequent request.
+ // See Issue 11549.
+- if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF {
++ if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF.isSet() {
+ w.closeAfterReply = true
+ }
+
+@@ -1561,6 +1578,17 @@ func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err er
+ }
+ return 0, ErrHijacked
+ }
++
++ if w.canWriteContinue.isSet() {
++ // Body reader wants to write 100 Continue but hasn't yet.
++ // Tell it not to. The store must be done while holding the lock
++ // because the lock makes sure that there is not an active write
++ // this very moment.
++ w.writeContinueMu.Lock()
++ w.canWriteContinue.setFalse()
++ w.writeContinueMu.Unlock()
++ }
++
+ if !w.wroteHeader {
+ w.WriteHeader(StatusOK)
+ }
+@@ -1872,6 +1900,7 @@ func (c *conn) serve(ctx context.Context) {
+ if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
+ // Wrap the Body reader with one that replies on the connection
+ req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
++ w.canWriteContinue.setTrue()
+ }
+ } else if req.Header.get("Expect") != "" {
+ w.sendExpectationFailed()
+--
+2.17.1
+
