ssh: fix deadlock on unexpected global responses

Previously, the mux implementation handled global request responses by
blocking until the response could be sent to the globalResponses channel.
Since this channel has a buffer size of 1, unsolicited responses from a
server (or responses arriving after a timeout) would fill the buffer.
Subsequent unsolicited responses would block handleGlobalPacket, stalling
the entire connection's read loop and causing a denial of service.

This change modifies handleGlobalPacket to use a non-blocking send. If
no goroutine is waiting for a response (or the buffer is full), the
message is dropped. This aligns with OpenSSH behavior, which ignores
unexpected global responses.

Additionally, SendRequest now drains the globalResponses channel after
acquiring the mutex but before sending the request. This ensures that
any stale responses or "spam" buffered just before the lock was acquired
are discarded, preventing race conditions where a legitimate request
might otherwise consume an unrelated response.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport.

Fixes golang/go#79564
Fixes CVE-2026-39830

Change-Id: Ia0c46355203d557eadcd432c10b87c8a044e1089
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781640
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/mux.go

@@ -91,9 +91,10 @@ type mux struct {
 
 	incomingChannels chan NewChannel
 
-	globalSentMu     sync.Mutex
-	globalResponses  chan interface{}
-	incomingRequests chan *Request
+	globalSentMu      sync.Mutex
+	globalSentPending atomic.Bool
+	globalResponses   chan interface{}
+	incomingRequests  chan *Request
 
 	errCond *sync.Cond
 	err     error
@@ -141,6 +142,24 @@ func (m *mux) SendRequest(name string, wantReply bool, payload []byte) (bool, []
 	if wantReply {
 		m.globalSentMu.Lock()
 		defer m.globalSentMu.Unlock()
+
+		// Open the gate so that responses arriving while this request is in
+		// flight are allowed to reach globalResponses. Any response arriving
+		// while no request is pending is dropped by handleGlobalPacket.
+		m.globalSentPending.Store(true)
+		defer m.globalSentPending.Store(false)
+
+		// Drain any spurious responses that may have been buffered. This prevents
+		// a previously buffered unexpected response from being consumed instead
+		// of the actual response for this request.
+	drain:
+		for {
+			select {
+			case <-m.globalResponses:
+			default:
+				break drain
+			}
+		}
 	}
 
 	if err := m.sendMessage(globalRequestMsg{
@@ -267,7 +286,16 @@ func (m *mux) handleGlobalPacket(packet []byte) error {
 			mux:       m,
 		}
 	case *globalRequestSuccessMsg, *globalRequestFailureMsg:
-		m.globalResponses <- msg
+		// Drop responses that arrive when no SendRequest is waiting, to
+		// prevent a malicious peer from staging responses for a future
+		// caller.
+		if !m.globalSentPending.Load() {
+			return nil
+		}
+		select {
+		case m.globalResponses <- msg:
+		default:
+		}
 	default:
 		panic(fmt.Sprintf("not a global message %#v", msg))
 	}