ssh: prevent malformed exit-status panic

A malicious server can crash any client by sending
a truncated exit-status payload, which triggers a
panic in binary.BigEndian.Uint32.

Change-Id: Iff4fe182b40210c99d46562f6c3b5ecf38bb98b4
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/780181
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>

ssh/session.go

@@ -423,6 +423,9 @@ func (s *Session) wait(reqs <-chan *Request) error {
 	for msg := range reqs {
 		switch msg.Type {
 		case "exit-status":
+			if len(msg.Payload) < 4 {
+				return errors.New("ssh: malformed exit-status request")
+			}
 			wm.status = int(binary.BigEndian.Uint32(msg.Payload))
 		case "exit-signal":
 			var sigval struct {