AUTH-2014: Checks users login shell

sshserver/get_user.go

@@ -0,0 +1,227 @@
+// Taken from https://github.com/golang/go/blob/ad644d2e86bab85787879d41c2d2aebbd7c57db8/src/os/user/user.go
+// and modified to return login shell in User struct. cloudflared requires cgo for compilation because of this addition.
+
+
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd !android,linux netbsd openbsd solaris
+// +build cgo,!osusergo
+
+package sshserver
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"syscall"
+	"unsafe"
+)
+
+/*
+#cgo solaris CFLAGS: -D_POSIX_PTHREAD_SEMANTICS
+#include <unistd.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <grp.h>
+#include <stdlib.h>
+
+static int mygetpwuid_r(int uid, struct passwd *pwd,
+	char *buf, size_t buflen, struct passwd **result) {
+	return getpwuid_r(uid, pwd, buf, buflen, result);
+}
+
+static int mygetpwnam_r(const char *name, struct passwd *pwd,
+	char *buf, size_t buflen, struct passwd **result) {
+	return getpwnam_r(name, pwd, buf, buflen, result);
+}
+
+static int mygetgrgid_r(int gid, struct group *grp,
+	char *buf, size_t buflen, struct group **result) {
+ return getgrgid_r(gid, grp, buf, buflen, result);
+}
+
+static int mygetgrnam_r(const char *name, struct group *grp,
+	char *buf, size_t buflen, struct group **result) {
+ return getgrnam_r(name, grp, buf, buflen, result);
+}
+*/
+import "C"
+
+type UnknownUserIdError int
+
+func (e UnknownUserIdError) Error() string {
+	return "user: unknown userid " + strconv.Itoa(int(e))
+}
+
+// UnknownUserError is returned by Lookup when
+// a user cannot be found.
+type UnknownUserError string
+
+func (e UnknownUserError) Error() string {
+	return "user: unknown user " + string(e)
+}
+
+// UnknownGroupIdError is returned by LookupGroupId when
+// a group cannot be found.
+type UnknownGroupIdError string
+
+func (e UnknownGroupIdError) Error() string {
+	return "group: unknown groupid " + string(e)
+}
+
+// UnknownGroupError is returned by LookupGroup when
+// a group cannot be found.
+type UnknownGroupError string
+
+func (e UnknownGroupError) Error() string {
+	return "group: unknown group " + string(e)
+}
+
+type User struct {
+	// Uid is the user ID.
+	// On POSIX systems, this is a decimal number representing the uid.
+	// On Windows, this is a security identifier (SID) in a string format.
+	// On Plan 9, this is the contents of /dev/user.
+	Uid string
+	// Gid is the primary group ID.
+	// On POSIX systems, this is a decimal number representing the gid.
+	// On Windows, this is a SID in a string format.
+	// On Plan 9, this is the contents of /dev/user.
+	Gid string
+	// Username is the login name.
+	Username string
+	// Name is the user's real or display name.
+	// It might be blank.
+	// On POSIX systems, this is the first (or only) entry in the GECOS field
+	// list.
+	// On Windows, this is the user's display name.
+	// On Plan 9, this is the contents of /dev/user.
+	Name string
+	// HomeDir is the path to the user's home directory (if they have one).
+	HomeDir string
+
+	/****************** Begin added code ******************/
+	// Login shell
+	Shell string
+	/****************** End added code ******************/
+}
+
+func lookupUser(username string) (*User, error) {
+	var pwd C.struct_passwd
+	var result *C.struct_passwd
+	nameC := make([]byte, len(username)+1)
+	copy(nameC, username)
+
+	buf := alloc(userBuffer)
+	defer buf.free()
+
+	err := retryWithBuffer(buf, func() syscall.Errno {
+		// mygetpwnam_r is a wrapper around getpwnam_r to avoid
+		// passing a size_t to getpwnam_r, because for unknown
+		// reasons passing a size_t to getpwnam_r doesn't work on
+		// Solaris.
+		return syscall.Errno(C.mygetpwnam_r((*C.char)(unsafe.Pointer(&nameC[0])),
+			&pwd,
+			(*C.char)(buf.ptr),
+			C.size_t(buf.size),
+			&result))
+	})
+	if err != nil {
+		return nil, fmt.Errorf("user: lookup username %s: %v", username, err)
+	}
+	if result == nil {
+		return nil, UnknownUserError(username)
+	}
+	return buildUser(&pwd), err
+}
+
+func buildUser(pwd *C.struct_passwd) *User {
+	u := &User{
+		Uid:      strconv.FormatUint(uint64(pwd.pw_uid), 10),
+		Gid:      strconv.FormatUint(uint64(pwd.pw_gid), 10),
+		Username: C.GoString(pwd.pw_name),
+		Name:     C.GoString(pwd.pw_gecos),
+		HomeDir:  C.GoString(pwd.pw_dir),
+		/****************** Begin added code ******************/
+		Shell:    C.GoString(pwd.pw_shell),
+		/****************** End added code ******************/
+	}
+	// The pw_gecos field isn't quite standardized. Some docs
+	// say: "It is expected to be a comma separated list of
+	// personal data where the first item is the full name of the
+	// user."
+	if i := strings.Index(u.Name, ","); i >= 0 {
+		u.Name = u.Name[:i]
+	}
+	return u
+}
+
+type bufferKind C.int
+
+const (
+	userBuffer = bufferKind(C._SC_GETPW_R_SIZE_MAX)
+)
+
+func (k bufferKind) initialSize() C.size_t {
+	sz := C.sysconf(C.int(k))
+	if sz == -1 {
+		// DragonFly and FreeBSD do not have _SC_GETPW_R_SIZE_MAX.
+		// Additionally, not all Linux systems have it, either. For
+		// example, the musl libc returns -1.
+		return 1024
+	}
+	if !isSizeReasonable(int64(sz)) {
+		// Truncate.  If this truly isn't enough, retryWithBuffer will error on the first run.
+		return maxBufferSize
+	}
+	return C.size_t(sz)
+}
+
+type memBuffer struct {
+	ptr  unsafe.Pointer
+	size C.size_t
+}
+
+func alloc(kind bufferKind) *memBuffer {
+	sz := kind.initialSize()
+	return &memBuffer{
+		ptr:  C.malloc(sz),
+		size: sz,
+	}
+}
+
+func (mb *memBuffer) resize(newSize C.size_t) {
+	mb.ptr = C.realloc(mb.ptr, newSize)
+	mb.size = newSize
+}
+
+func (mb *memBuffer) free() {
+	C.free(mb.ptr)
+}
+
+// retryWithBuffer repeatedly calls f(), increasing the size of the
+// buffer each time, until f succeeds, fails with a non-ERANGE error,
+// or the buffer exceeds a reasonable limit.
+func retryWithBuffer(buf *memBuffer, f func() syscall.Errno) error {
+	for {
+		errno := f()
+		if errno == 0 {
+			return nil
+		} else if errno != syscall.ERANGE {
+			return errno
+		}
+		newSize := buf.size * 2
+		if !isSizeReasonable(int64(newSize)) {
+			return fmt.Errorf("internal buffer exceeds %d bytes", maxBufferSize)
+		}
+		buf.resize(newSize)
+	}
+}
+
+const maxBufferSize = 1 << 20
+
+func isSizeReasonable(sz int64) bool {
+	return sz > 0 && sz <= maxBufferSize
+}

sshserver/sshserver_unix.go

@@ -64,20 +64,17 @@ func (s *SSHServer) Start() error {
 func (s *SSHServer) connectionHandler(session ssh.Session) {
 
 	// Get uid and gid of user attempting to login
-	uid, gid, err := getUser(session.User())
+	sshUser, err := lookupUser(session.User())
 	if err != nil {
 		if _, err := io.WriteString(session, "Invalid credentials\n"); err != nil {
 			s.logger.WithError(err).Error("Invalid credentials: Failed to write to SSH session")
 		}
-		if err := session.Exit(1); err != nil {
-			s.logger.WithError(err).Error("Failed to close SSH session")
-		}
+		s.CloseSession(session)
 		return
 	}
 
 	// Spawn shell under user
-	cmd := exec.Command("/bin/bash")
-	cmd.SysProcAttr = &syscall.SysProcAttr{Credential: &syscall.Credential{Uid: uid, Gid: gid}}
+	cmd := exec.Command(sshUser.Shell)
 
 	ptyReq, winCh, isPty := session.Pty()
 	if !isPty {
@@ -85,20 +82,27 @@ func (s *SSHServer) connectionHandler(session ssh.Session) {
 			s.logger.WithError(err).Error("No PTY requested: Failed to write to SSH session")
 		}
 
-		if err := session.Exit(1); err != nil {
-			s.logger.WithError(err).Error("Failed to close SSH session")
-		}
+		s.CloseSession(session)
 		return
 	}
 
+	uidInt, uidErr := stringToUint32(sshUser.Uid)
+	gidInt, gidErr := stringToUint32(sshUser.Gid)
+	if uidErr != nil || gidErr != nil {
+		s.logger.WithError(err).Error("Invalid user")
+		s.CloseSession(session)
+		return
+	}
+
+	cmd.SysProcAttr = &syscall.SysProcAttr{Credential: &syscall.Credential{Uid: uidInt, Gid: gidInt}}
 	cmd.Env = append(cmd.Env, fmt.Sprintf("TERM=%s", ptyReq.Term))
-	cmd.Env = append(cmd.Env, fmt.Sprintf("PS1=%s", defaultShellPrompt))
+	cmd.Env = append(cmd.Env, fmt.Sprintf("USER=%s", sshUser.Name))
+	cmd.Env = append(cmd.Env, fmt.Sprintf("HOME=%s", sshUser.HomeDir))
+	cmd.Dir = sshUser.HomeDir
 	psuedoTTY, err := pty.Start(cmd)
 	if err != nil {
 		s.logger.WithError(err).Error("Failed to start pty session")
-		if err := session.Exit(1); err != nil {
-			s.logger.WithError(err).Error("Failed to close SSH session")
-		}
+		s.CloseSession(session)
 		close(s.shutdownC)
 		return
 	}
@@ -108,9 +112,7 @@ func (s *SSHServer) connectionHandler(session ssh.Session) {
 		for win := range winCh {
 			if errNo := setWinsize(psuedoTTY, win.Width, win.Height); errNo != 0 {
 				s.logger.WithError(err).Error("Failed to set pty window size: ", err.Error())
-				if err := session.Exit(1); err != nil {
-					s.logger.WithError(err).Error("Failed to close SSH session")
-				}
+				s.CloseSession(session)
 				close(s.shutdownC)
 				return
 			}
@@ -152,6 +154,12 @@ func (s *SSHServer) connectionHandler(session ssh.Session) {
 	}
 }
 
+func (s *SSHServer) CloseSession(session ssh.Session) {
+	if err := session.Exit(1); err != nil {
+		s.logger.WithError(err).Error("Failed to close SSH session")
+	}
+}
+
 // Sets PTY window size for terminal
 func setWinsize(f *os.File, w, h int) syscall.Errno {
 	_, _, errNo := syscall.Syscall(syscall.SYS_IOCTL, f.Fd(), uintptr(syscall.TIOCSWINSZ),
@@ -159,19 +167,8 @@ func setWinsize(f *os.File, w, h int) syscall.Errno {
 	return errNo
 }
 
-// Only works on POSIX systems
-func getUser(username string) (uint32, uint32, error) {
-	sshUser, err := user.Lookup(username)
-	if err != nil {
-		return 0, 0, err
-	}
-	uid, err := strconv.ParseUint(sshUser.Uid, 10, 32)
-	if err != nil {
-		return 0, 0, err
-	}
-	gid, err := strconv.ParseUint(sshUser.Gid, 10, 32)
-	if err != nil {
-		return 0, 0, err
-	}
-	return uint32(uid), uint32(gid), nil
+func stringToUint32(str string) (uint32, error) {
+	uid, err := strconv.ParseUint(str, 10, 32)
+	return uint32(uid), err
+
 }