fix: allow concurrent SFTP connections (#18013)

current implementation did not fully implement
the concurrent SFTP connection implementation,
this PR properly handles this.

fixes #17914

cmd/ftp-server.go

@@ -21,11 +21,11 @@ import (
 	"context"
 	"crypto/subtle"
 	"fmt"
-	"io"
 	"net"
 	"os"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/minio/cli"
 	"github.com/minio/minio/internal/logger"
@@ -75,6 +75,69 @@ func (log *minioLogger) PrintResponse(sessionID string, code int, message string
 	}
 }
 
+func acceptSFTPConnection(conn net.Conn, config *ssh.ServerConfig) {
+	// For SSH handshake keep a 2 minute deadline, as OpensSSH default.
+	conn.SetDeadline(time.Now().Add(2 * time.Minute))
+
+	// Before use, a handshake must be performed on the incoming net.Conn.
+	sconn, chans, reqs, err := ssh.NewServerConn(conn, config)
+	if err != nil {
+		return
+	}
+
+	// Once we are done with SSH handshake, remove deadline.
+	conn.SetDeadline(time.Time{})
+
+	// The incoming Request channel must be serviced.
+	go ssh.DiscardRequests(reqs)
+
+	// Service the incoming Channel channel.
+	for newChannel := range chans {
+		// Channels have a type, depending on the application level
+		// protocol intended. In the case of an SFTP session, this is "subsystem"
+		// with a payload string of "<length=4>sftp"
+		if newChannel.ChannelType() != "session" {
+			newChannel.Reject(ssh.UnknownChannelType, "unknown channel type")
+			continue
+		}
+
+		channel, requests, err := newChannel.Accept()
+		if err != nil {
+			logger.LogOnceIf(context.Background(), fmt.Errorf("unable to accept the connection requests channel: %w", err), "accept-channel-sftp")
+			continue
+		}
+
+		// Sessions have out-of-band requests such as "shell",
+		// "pty-req" and "env".  Here we handle only the
+		// "subsystem" request.
+		go func(in <-chan *ssh.Request) {
+			for req := range in {
+				ok := false
+				if req.Type == "subsystem" {
+					if len(req.Payload) > 4 && string(req.Payload[4:]) == "sftp" {
+						ok = true
+						go handleSFTPConnection(channel, sconn)
+					}
+				}
+
+				if req.WantReply {
+					// We only reply to SSH packets that have `sftp` payload, all other
+					// packets are rejected
+					req.Reply(ok, nil)
+				}
+			}
+		}(requests)
+	}
+}
+
+func handleSFTPConnection(channel ssh.Channel, sconn *ssh.ServerConn) {
+	// Create the server instance for the channel using the handler we created above.
+	server := sftp.NewRequestServer(channel, NewSFTPDriver(sconn.Permissions), sftp.WithRSAllocator())
+	defer server.Close()
+
+	server.Serve()
+}
+
 func startSFTPServer(c *cli.Context) {
 	args := c.StringSlice("sftp")
 
@@ -176,54 +239,29 @@ func startSFTPServer(c *cli.Context) {
 
 	logger.Info(fmt.Sprintf("MinIO SFTP Server listening on %s", net.JoinHostPort(publicIP, strconv.Itoa(port))))
 
+	var tempDelay time.Duration // how long to sleep on accept failure
+
 	for {
-		nConn, err := listener.Accept()
+		conn, err := listener.Accept()
 		if err != nil {
-			logger.LogIf(context.Background(), err)
+			// From https://github.com/golang/go/blob/4aa1efed4853ea067d665a952eee77c52faac774/src/net/http/server.go#L3046
+			if ne, ok := err.(net.Error); ok && ne.Temporary() {
+				if tempDelay == 0 {
+					tempDelay = 5 * time.Millisecond
+				} else {
+					tempDelay *= 2
+				}
+				if max := 1 * time.Second; tempDelay > max {
+					tempDelay = max
+				}
+				logger.LogOnceIf(context.Background(), fmt.Errorf("error while accepting connections: %w, retrying in %s", err, tempDelay), "accept-limit-sftp")
+				time.Sleep(tempDelay)
 				continue
 			}
-
+			logger.Fatal(err, "unrecoverable error while accepting new connections")
-		// Before use, a handshake must be performed on the incoming net.Conn.
-		sconn, chans, reqs, err := ssh.NewServerConn(nConn, config)
-		if err != nil {
-			logger.LogIf(context.Background(), err)
-			continue
 		}
 
-		// The incoming Request channel must be serviced.
+		go acceptSFTPConnection(conn, config)
-		go ssh.DiscardRequests(reqs)
-
-		// Service the incoming Channel channel.
-		for newChannel := range chans {
-			// Channels have a type, depending on the application level
-			// protocol intended. In the case of an SFTP session, this is "subsystem"
-			// with a payload string of "<length=4>sftp"
-			if newChannel.ChannelType() != "session" {
-				newChannel.Reject(ssh.UnknownChannelType, "unknown channel type")
-				continue
-			}
-			channel, requests, err := newChannel.Accept()
-			if err != nil {
-				logger.Fatal(err, "unable to accept the connection requests channel")
-			}
-
-			// Sessions have out-of-band requests such as "shell",
-			// "pty-req" and "env".  Here we handle only the
-			// "subsystem" request.
-			go func(in <-chan *ssh.Request) {
-				for req := range in {
-					// We only reply to SSH packets that have `sftp` payload.
-					req.Reply(req.Type == "subsystem" && string(req.Payload[4:]) == "sftp", nil)
-				}
-			}(requests)
-
-			server := sftp.NewRequestServer(channel, NewSFTPDriver(sconn.Permissions))
-			if err := server.Serve(); err == io.EOF {
-				server.Close()
-			} else if err != nil {
-				logger.Fatal(err, "unable to start SFTP server")
-			}
-		}
 	}
 }