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@@ -0,0 +1,131 @@
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+From fa98f46741f818913a8c11b877520a548715131f Mon Sep 17 00:00:00 2001
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+From: Russ Cox <rsc@golang.org>
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+Date: Mon, 13 Jul 2020 13:27:22 -0400
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+Subject: [PATCH] net/http: synchronize "100 Continue" write and Handler writes
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+
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+The expectContinueReader writes to the connection on the first
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+Request.Body read. Since a Handler might be doing a read in parallel or
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+before a write, expectContinueReader needs to synchronize with the
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+ResponseWriter, and abort if a response already went out.
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+
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+The tests will land in a separate CL.
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+
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+Fixes #34902
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+Fixes CVE-2020-15586
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+
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+Change-Id: Icdd8dd539f45e8863762bd378194bb4741e875fc
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+Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/793350
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+Reviewed-by: Filippo Valsorda <valsorda@google.com>
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+Reviewed-on: https://go-review.googlesource.com/c/go/+/242598
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+Run-TryBot: Katie Hockman <katie@golang.org>
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+Reviewed-by: Filippo Valsorda <filippo@golang.org>
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+TryBot-Result: Gobot Gobot <gobot@golang.org>
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+
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+Upstream-Status: Backport
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+CVE: CVE-2020-15586
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+Signed-off-by: Li Zhou <li.zhou@windriver.com>
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+---
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+ src/net/http/server.go | 43 +++++++++++++++++++++++++++++++++++-------
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+ 1 file changed, 36 insertions(+), 7 deletions(-)
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+
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+diff --git a/src/net/http/server.go b/src/net/http/server.go
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+index a995a50658..d41b5f6f48 100644
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+--- a/src/net/http/server.go
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++++ b/src/net/http/server.go
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+@@ -425,6 +425,16 @@ type response struct {
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+ wants10KeepAlive bool // HTTP/1.0 w/ Connection "keep-alive"
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+ wantsClose bool // HTTP request has Connection "close"
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+
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++ // canWriteContinue is a boolean value accessed as an atomic int32
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++ // that says whether or not a 100 Continue header can be written
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++ // to the connection.
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++ // writeContinueMu must be held while writing the header.
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++ // These two fields together synchronize the body reader
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++ // (the expectContinueReader, which wants to write 100 Continue)
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++ // against the main writer.
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++ canWriteContinue atomicBool
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++ writeContinueMu sync.Mutex
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++
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+ w *bufio.Writer // buffers output in chunks to chunkWriter
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+ cw chunkWriter
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+
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+@@ -515,6 +525,7 @@ type atomicBool int32
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+
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+ func (b *atomicBool) isSet() bool { return atomic.LoadInt32((*int32)(b)) != 0 }
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+ func (b *atomicBool) setTrue() { atomic.StoreInt32((*int32)(b), 1) }
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++func (b *atomicBool) setFalse() { atomic.StoreInt32((*int32)(b), 0) }
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+
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+ // declareTrailer is called for each Trailer header when the
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+ // response header is written. It notes that a header will need to be
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+@@ -878,21 +889,27 @@ type expectContinueReader struct {
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+ resp *response
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+ readCloser io.ReadCloser
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+ closed bool
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+- sawEOF bool
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++ sawEOF atomicBool
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+ }
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+
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+ func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
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+ if ecr.closed {
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+ return 0, ErrBodyReadAfterClose
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+ }
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+- if !ecr.resp.wroteContinue && !ecr.resp.conn.hijacked() {
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+- ecr.resp.wroteContinue = true
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+- ecr.resp.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
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+- ecr.resp.conn.bufw.Flush()
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++ w := ecr.resp
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++ if !w.wroteContinue && w.canWriteContinue.isSet() && !w.conn.hijacked() {
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++ w.wroteContinue = true
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++ w.writeContinueMu.Lock()
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++ if w.canWriteContinue.isSet() {
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++ w.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
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++ w.conn.bufw.Flush()
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++ w.canWriteContinue.setFalse()
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++ }
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++ w.writeContinueMu.Unlock()
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+ }
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+ n, err = ecr.readCloser.Read(p)
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+ if err == io.EOF {
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+- ecr.sawEOF = true
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++ ecr.sawEOF.setTrue()
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+ }
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+ return
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+ }
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+@@ -1311,7 +1328,7 @@ func (cw *chunkWriter) writeHeader(p []byte) {
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+ // because we don't know if the next bytes on the wire will be
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+ // the body-following-the-timer or the subsequent request.
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+ // See Issue 11549.
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+- if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF {
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++ if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF.isSet() {
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+ w.closeAfterReply = true
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+ }
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+
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+@@ -1561,6 +1578,17 @@ func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err er
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+ }
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+ return 0, ErrHijacked
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+ }
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++
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++ if w.canWriteContinue.isSet() {
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++ // Body reader wants to write 100 Continue but hasn't yet.
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++ // Tell it not to. The store must be done while holding the lock
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++ // because the lock makes sure that there is not an active write
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++ // this very moment.
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++ w.writeContinueMu.Lock()
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++ w.canWriteContinue.setFalse()
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++ w.writeContinueMu.Unlock()
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++ }
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++
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+ if !w.wroteHeader {
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+ w.WriteHeader(StatusOK)
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+ }
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+@@ -1872,6 +1900,7 @@ func (c *conn) serve(ctx context.Context) {
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+ if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
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+ // Wrap the Body reader with one that replies on the connection
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+ req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
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++ w.canWriteContinue.setTrue()
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+ }
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+ } else if req.Header.get("Expect") != "" {
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+ w.sendExpectationFailed()
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+--
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+2.17.1
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+
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