x/crypto/ssh: Dial hangs in kexLoop indefinitely - ignoring ClientConfig.Timeout

[pjbgf]

What version of Go are you using (go version)?

$ go version
go version go1.17.8 linux/amd64

Does this issue reproduce with the latest release?

Yes, as this is library related using version:

golang.org/x/crypto v0.0.0-20220321153916-2c7772ba3064

I can confirm the issue also happens with previous versions:

golang.org/x/[email protected]

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/USER/.cache/go-build"
GOENV="/home/USER/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/USER/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/USER/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.8"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build762703969=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The application implements a golang ssh transport that hangs indefinitely at ssh.Dial every so often.
The current timeout is set to 30 seconds, which ssh.Dial does not uphold (https://github.com/fluxcd/source-controller/blob/main/pkg/git/libgit2/managed/ssh.go#L251-L255 https://github.com/fluxcd/source-controller/blob/main/pkg/git/libgit2/managed/init.go#L30).

This is a low concurrency (2-4 parallel workers) application which creates multiple ssh connections to execute simple git operations.

The ssh.Dial uses the ssh.ClientConfig as below:

        ssh.ClientConfig{
		User:    username,
		Auth:    []ssh.AuthMethod{ssh.PublicKeys(key)},
		Timeout: 30 * time.Second,
	}

Actual code can be seen at:
https://github.com/fluxcd/source-controller/blob/main/pkg/git/libgit2/managed/ssh.go#L166

What did you expect to see?

The ssh.Dial operation error if the Dial operation took longer than the pre-configured timeout.

What did you see instead?

The goroutine hangs indefinitely. pprof shows the culprit being:

goroutine 25748 [select, 50 minutes]:
golang.org/x/crypto/ssh.(*handshakeTransport).kexLoop(0xc0003398c0)
	golang.org/x/[email protected]/ssh/handshake.go:268 +0x485
created by golang.org/x/crypto/ssh.newClientTransport
	golang.org/x/[email protected]/ssh/handshake.go:135 +0x23d

[mknyszek]

CC @FiloSottile

[pjbgf]

I have noticed that this seems to happen only on subsequent/concurrent connections. I could not reproduce it ever happening on the first connection. So potentially this is somehow related to #27140.

[pjbgf]

By ensuring the session's StdoutPipe is serviced quickly, seems to resolve the problem, as mentioned on crypto/ssh comments:

https://github.com/golang/crypto/blob/eb4f295cb31f7fb5d52810411604a2638c9b19a2/ssh/session.go#L553-L558

[mvo5]

I ran into the issue that ssh.Dial() hangs forever too and I wonder if something like:

diff --git a/ssh/client.go b/ssh/client.go
index 6fd1994..a1300eb 100644
--- a/ssh/client.go
+++ b/ssh/client.go
@@ -174,6 +174,14 @@ func Dial(network, addr string, config *ClientConfig) (*Client, error) {
        if err != nil {
                return nil, err
        }
+       if config.Timeout > 0 {
+               if err := conn.SetDeadline(time.Now().Add(config.Timeout)); err != nil {
+                       return nil, err
+               }
+               defer func() {
+                       conn.SetDeadline(time.Time{})
+               }()
+       }
        c, chans, reqs, err := NewClientConn(conn, addr, config)
        if err != nil {
                return nil, err
@@ -231,7 +239,7 @@ type ClientConfig struct {
        // any of the CertAlgoXxxx and KeyAlgoXxxx constants.
        HostKeyAlgorithms []string
 
-       // Timeout is the maximum amount of time for the TCP connection to establish.
+       // Timeout is the maximum amount of time to establish the connection.
        //
        // A Timeout of zero means no timeout.
        Timeout time.Duration

is a reasonable way to address it? It fixes the issue for me with the hangs I was seeing.

If there is a chance that upstream is interested I could do a PR with a proper test etc but
of course there are questions as the semantic (slightly) changes and the timeout now encompasses
both the TCP connection and the initial setup plus it's actually two timeouts now so the updated comment
may be misleading.