Repository: go
Issue: x/crypto/ssh: SSH handshake fails over some net.Conn implementations (ex. net.Pipe)
Labels: help wanted,NeedsInvestigation

### What version of Go are you using (`go version`)?

Tested on:
<pre>
go version go1.12 linux/amd64
go version go1.12.4 windows/amd64
</pre>

### Does this issue reproduce with the latest release?
Yes

### What operating system and processor architecture are you using (`go env`)?
linux/windows amd64

### What did you do?

Run the following unit test:
```$ go test -timeout 30s sshproxy -run TestHandshakeOverPipe```
```go
package sshproxy

import (
	"log"
	"net"
	"testing"

	"github.com/pkg/errors"
	"golang.org/x/crypto/ssh"
)

const (
	testUserName = "test-user"
	testUserPass = "test-password"
)

var testServerCfg = &ssh.ServerConfig{
	PasswordCallback: func(c ssh.ConnMetadata, pass []byte) (*ssh.Permissions, error) {
		if c.User() == testUserName && string(pass) == testUserPass {
			return nil, nil
		}
		return nil, errors.Errorf("User %q supplied an incorrect password", c.User())
	},
}

var testClientCfg = &ssh.ClientConfig{
	User: testUserName,
	Auth: []ssh.AuthMethod{
		ssh.Password(testUserPass),
	},
	HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
		return nil
	},
}

func init() {
	privateKey, err := ssh.ParsePrivateKey([]byte(testPrivateKey))
	if err != nil {
		log.Fatalf("Unit test init: Failed to parse private key; Details: %s", err)
	}
	testServerCfg.AddHostKey(privateKey)
}

func TestHandshakeOverPipe(t *testing.T) {
	srvConn, cliConn := net.Pipe()
	defer srvConn.Close()
	defer cliConn.Close()	

	errCh := make(chan error, 1)
	go func() {
		_, _, _, err := ssh.NewServerConn(srvConn, testServerCfg)
		errCh <- err
	}()

	if _, _, _, err := ssh.NewClientConn(cliConn, "localhost", testClientCfg); err != nil {
		t.Fatalf("Client SSH handshake failed; Details: %s", err)
	} else if err = <-errCh; err != nil {
		t.Fatalf("Server SSH handshake failed; Details: %s", err)
	}
}

//redacted
//const testPrivateKey = ... 
```

### What did you expect to see?

Test pass (_test passes when modified to use a real TCP connection_)

### What did you see instead?

Tests hang with both client and sever writing to write to the ``net.Conn`` during ``ssh.exchangeVersions`` called from ``clientHandshake`` and ``serverHandshake`` respectively.

```
$ go test -timeout 30s sshproxy -run TestHandshakeOverPipe
panic: test timed out after 30s

goroutine 18 [running]:
testing.(*M).startAlarm.func1()
        /usr/local/go/src/testing/testing.go:1334 +0xdf
created by time.goFunc
        /usr/local/go/src/time/sleep.go:169 +0x44

goroutine 1 [chan receive]:
testing.(*T).Run(0xc0000ee100, 0x659a66, 0x15, 0x663b30, 0x476aa6)
        /usr/local/go/src/testing/testing.go:917 +0x37e
testing.runTests.func1(0xc0000ee000)
        /usr/local/go/src/testing/testing.go:1157 +0x78
testing.tRunner(0xc0000ee000, 0xc0000a7e30)
        /usr/local/go/src/testing/testing.go:865 +0xc0
testing.runTests(0xc00000ed00, 0x80c9a0, 0x2, 0x2, 0x0)
        /usr/local/go/src/testing/testing.go:1155 +0x2a9
testing.(*M).Run(0xc0000d8000, 0x0)
        /usr/local/go/src/testing/testing.go:1072 +0x162
main.main()
        _testmain.go:44 +0x13e

goroutine 6 [select]:
net.(*pipe).write(0xc0000d8100, 0xc0000183a0, 0xc, 0x20, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/pipe.go:199 +0x27a
net.(*pipe).Write(0xc0000d8100, 0xc0000183a0, 0xc, 0x20, 0xc, 0xc0000183a0, 0xa)
        /usr/local/go/src/net/pipe.go:179 +0x4d
golang.org/x/crypto/ssh.exchangeVersions(0x7f95454b5060, 0xc0000d8100, 0xc000016520, 0xa, 0xa, 0x8, 0x4, 0x4, 0xa, 0xc000042660)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/transport.go:297 +0xef
golang.org/x/crypto/ssh.(*connection).clientHandshake(0xc0000d8200, 0x656b21, 0x9, 0xc00008eb60, 0xc000082a01, 0xc00006a360)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/client.go:100 +0xe7
golang.org/x/crypto/ssh.NewClientConn(0x69e200, 0xc0000d8100, 0x656b21, 0x9, 0x810da0, 0xc0000128c0, 0x24, 0x31d, 0x4cd7d0, 0x1, ...)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/client.go:83 +0xfe
sshproxy.TestHandshakeOverPipe(0xc0000ee100)
        sshproxy/tunnel_test.go:60 +0xf3
testing.tRunner(0xc0000ee100, 0x663b30)
        /usr/local/go/src/testing/testing.go:865 +0xc0
created by testing.(*T).Run
        /usr/local/go/src/testing/testing.go:916 +0x357

goroutine 7 [select]:
net.(*pipe).write(0xc0000d8080, 0xc0000183c0, 0xc, 0x20, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/pipe.go:199 +0x27a
net.(*pipe).Write(0xc0000d8080, 0xc0000183c0, 0xc, 0x20, 0xc, 0xc0000183c0, 0xa)
        /usr/local/go/src/net/pipe.go:179 +0x4d
golang.org/x/crypto/ssh.exchangeVersions(0x7f95454b5060, 0xc0000d8080, 0xc000016530, 0xa, 0xa, 0xc000042ef8, 0x78, 0x78, 0xc0000d8300, 0x4)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/transport.go:297 +0xef
golang.org/x/crypto/ssh.(*connection).serverHandshake(0xc0000d8300, 0xc0000f2000, 0x0, 0x0, 0x0)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/server.go:217 +0x136
golang.org/x/crypto/ssh.NewServerConn(0x69e200, 0xc0000d8080, 0x8109e0, 0x0, 0x0, 0x0, 0x0, 0x0)
        go/pkg/mod/golang.org/x/crypto@v0.0.0-20190308221718-c2843e01d9a2/ssh/server.go:182 +0xd6
sshproxy.TestHandshakeOverPipe.func1(0x69e200, 0xc0000d8080, 0xc00006a360)
        sshproxy/tunnel_test.go:56 +0x49
created by sshproxy.TestHandshakeOverPipe
        sshproxy/tunnel_test.go:55 +0xaf
FAIL    sshproxy  30.012s
```