REPL Server: Guard against HTTP inter-protocol exploitation attacks.

Reported by Christopher Allan Webber <cwebber@dustycloud.org>
Co-authored-by: Ludovic Courtès <ludo@gnu.org>

This commit adds protection to Guile's REPL servers against HTTP
inter-protocol exploitation attacks, a scenario whereby an attacker can,
via an HTML page, cause a web browser to send data to TCP servers
listening on a loopback interface or private network.  See
<https://en.wikipedia.org/wiki/Inter-protocol_exploitation> and
<https://www.jochentopf.com/hfpa/hfpa.pdf>, The HTML Form Protocol
Attack (2001) by Tochen Topf <jochen@remote.org>.

Here we add a procedure to 'before-read-hook' that looks for a possible
HTTP request-line in the first line of input from the client socket.  If
present, the socket is drained and closed, and a loud warning is written
to stderr (POSIX file descriptor 2).

* module/system/repl/server.scm: Add 'maybe-check-for-http-request'
to 'before-read-hook' when this module is loaded.
(with-temporary-port-encoding, with-saved-port-line+column)
(drain-input-and-close, permissive-http-request-line?)
(check-for-http-request, guard-against-http-request)
(maybe-check-for-http-request): New procedures.
(serve-client): Use 'guard-against-http-request'.
* module/system/repl/coop-server.scm (start-repl-client): Use
'guard-against-http-request'.
* doc/ref/guile-invoke.texi (Command-line Options): In the description
of the --listen option, make the security warning more prominent.
Mention the new protection added here.  Recommend using UNIX domain
sockets for REPL servers.  "a path to" => "the file name of".

doc/ref/guile-invoke.texi

@@ -1,7 +1,7 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2010, 2011, 2013, 2014
-@c   Free Software Foundation, Inc.
+@c Copyright (C)  1996, 1997, 2000-2005, 2010, 2011, 2013, 2014,
+@c   2016 Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
 @node Invoking Guile
@@ -176,7 +176,7 @@ the @file{.guile} file.  @xref{Init File}.
 While this program runs, listen on a local port or a path for REPL
 clients.  If @var{p} starts with a number, it is assumed to be a local
 port on which to listen.  If it starts with a forward slash, it is
-assumed to be a path to a UNIX domain socket on which to listen.
+assumed to be the file name of a UNIX domain socket on which to listen.
 
 If @var{p} is not given, the default is local port 37146.  If you look
 at it upside down, it almost spells ``Guile''.  If you have netcat
@@ -184,12 +184,22 @@ installed, you should be able to @kbd{nc localhost 37146} and get a
 Guile prompt.  Alternately you can fire up Emacs and connect to the
 process; see @ref{Using Guile in Emacs} for more details.
 
-Note that opening a port allows anyone who can connect to that port---in
-the TCP case, any local user---to do anything Guile can do, as the user
+@quotation Note
+Opening a port allows anyone who can connect to that port to do anything
+Guile can do, as the user
 that the Guile process is running as.  Do not use @option{--listen} on
 multi-user machines.  Of course, if you do not pass @option{--listen} to
 Guile, no port will be opened.
 
+Guile protects against the
+@uref{https://en.wikipedia.org/wiki/Inter-protocol_exploitation,
+@dfn{HTTP inter-protocol exploitation attack}}, a scenario whereby an
+attacker can, @i{via} an HTML page, cause a web browser to send data to
+TCP servers listening on a loopback interface or private network.
+Nevertheless, you are advised to use UNIX domain sockets, as in
+@code{--listen=/some/local/file}, whenever possible.
+@end quotation
+
 That said, @option{--listen} is great for interactive debugging and
 development.
 

module/system/repl/coop-server.scm

@@ -1,6 +1,6 @@
 ;;; Cooperative REPL server
 
-;; Copyright (C) 2014 Free Software Foundation, Inc.
+;; Copyright (C) 2014, 2016 Free Software Foundation, Inc.
 
 ;; This library is free software; you can redistribute it and/or
 ;; modify it under the terms of the GNU Lesser General Public
@@ -25,7 +25,6 @@
   #:use-module (ice-9 threads)
   #:use-module (ice-9 q)
   #:use-module (srfi srfi-9)
-  #:use-module ((system repl server) #:select (make-tcp-server-socket))
   #:export (spawn-coop-repl-server
             poll-coop-repl-server))
 
@@ -35,7 +34,9 @@
     (define sym (@@ module sym))
     ...))
 (import-private (system repl repl) start-repl* prompting-meta-read)
-(import-private (system repl server) run-server* add-open-socket! close-socket!)
+(import-private (system repl server)
+                run-server* add-open-socket! close-socket!
+                make-tcp-server-socket guard-against-http-request)
 
 (define-record-type <coop-repl-server>
   (%make-coop-repl-server mutex queue)
@@ -177,6 +178,8 @@ and output is sent over the socket CLIENT."
   ;; another thread.
   (add-open-socket! client (lambda () (close-fdes (fileno client))))
 
+  (guard-against-http-request client)
+
   (with-continuation-barrier
    (lambda ()
      (coop-repl-prompt

module/system/repl/server.scm

@@ -1,6 +1,6 @@
 ;;; Repl server
 
-;; Copyright (C)  2003, 2010, 2011, 2014 Free Software Foundation, Inc.
+;; Copyright (C)  2003, 2010, 2011, 2014, 2016 Free Software Foundation, Inc.
 
 ;; This library is free software; you can redistribute it and/or
 ;; modify it under the terms of the GNU Lesser General Public
@@ -22,8 +22,14 @@
 (define-module (system repl server)
   #:use-module (system repl repl)
   #:use-module (ice-9 threads)
+  #:use-module (ice-9 rdelim)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 iconv)
+  #:use-module (rnrs bytevectors)
+  #:use-module (ice-9 binary-ports)
+  #:use-module ((rnrs io ports) #:select (call-with-port))
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)           ; cut
   #:export (make-tcp-server-socket
             make-unix-domain-server-socket
             run-server
@@ -136,6 +142,8 @@
     ;; To shut down this thread and socket, cause it to unwind.
     (add-open-socket! client (lambda () (cancel-thread thread))))
 
+  (guard-against-http-request client)
+
   (dynamic-wind
     (lambda () #f)
     (with-continuation-barrier
@@ -147,3 +155,176 @@
          (with-fluids ((*repl-stack* '()))
            (start-repl)))))
     (lambda () (close-socket! client))))
+
+
+;;;
+;;; The following code adds protection to Guile's REPL servers against
+;;; HTTP inter-protocol exploitation attacks, a scenario whereby an
+;;; attacker can, via an HTML page, cause a web browser to send data to
+;;; TCP servers listening on a loopback interface or private network.
+;;; See <https://en.wikipedia.org/wiki/Inter-protocol_exploitation> and
+;;; <https://www.jochentopf.com/hfpa/hfpa.pdf>, The HTML Form Protocol
+;;; Attack (2001) by Tochen Topf <jochen@remote.org>.
+;;;
+;;; Here we add a procedure to 'before-read-hook' that looks for a possible
+;;; HTTP request-line in the first line of input from the client socket.  If
+;;; present, the socket is drained and closed, and a loud warning is written
+;;; to stderr (POSIX file descriptor 2).
+;;;
+
+(define (with-temporary-port-encoding port encoding thunk)
+  "Call THUNK in a dynamic environment in which the encoding of PORT is
+temporarily set to ENCODING."
+  (let ((saved-encoding #f))
+    (dynamic-wind
+      (lambda ()
+        (unless (port-closed? port)
+          (set! saved-encoding (port-encoding port))
+          (set-port-encoding! port encoding)))
+      thunk
+      (lambda ()
+        (unless (port-closed? port)
+          (set! encoding (port-encoding port))
+          (set-port-encoding! port saved-encoding))))))
+
+(define (with-saved-port-line+column port thunk)
+  "Save the line and column of PORT before entering THUNK, and restore
+their previous values upon normal or non-local exit from THUNK."
+  (let ((saved-line #f) (saved-column #f))
+    (dynamic-wind
+      (lambda ()
+        (unless (port-closed? port)
+          (set! saved-line   (port-line   port))
+          (set! saved-column (port-column port))))
+      thunk
+      (lambda ()
+        (unless (port-closed? port)
+          (set-port-line!   port saved-line)
+          (set-port-column! port saved-column))))))
+
+(define (drain-input-and-close socket)
+  "Drain input from SOCKET using ISO-8859-1 encoding until it would block,
+and then close it.  Return the drained input as a string."
+  (dynamic-wind
+    (lambda ()
+      ;; Enable full buffering mode on the socket to allow
+      ;; 'get-bytevector-some' to return non-trivial chunks.
+      (setvbuf socket _IOFBF))
+    (lambda ()
+      (let loop ((chunks '()))
+        (let ((result (and (char-ready? socket)
+                           (get-bytevector-some socket))))
+          (if (bytevector? result)
+              (loop (cons (bytevector->string result "ISO-8859-1")
+                          chunks))
+              (string-concatenate-reverse chunks)))))
+    (lambda ()
+      ;; Close the socket even in case of an exception.
+      (close-port socket))))
+
+(define permissive-http-request-line?
+  ;; This predicate is deliberately permissive
+  ;; when checking the Request-URI component.
+  (let ((cs (ucs-range->char-set #x20 #x7E))
+        (rx (make-regexp
+             (string-append
+              "^(OPTIONS|GET|HEAD|POST|PUT|DELETE|TRACE|CONNECT) "
+              "[^ ]+ "
+              "HTTP/[0-9]+.[0-9]+$"))))
+    (lambda (line)
+      "Return true if LINE might plausibly be an HTTP request-line,
+otherwise return #f."
+      ;; We cannot simplify this to a simple 'regexp-exec', because
+      ;; 'regexp-exec' cannot cope with NUL bytes.
+      (and (string-every cs line)
+           (regexp-exec  rx line)))))
+
+(define (check-for-http-request socket)
+  "Check for a possible HTTP request in the initial input from SOCKET.
+If one is found, close the socket and print a report to STDERR (fdes 2).
+Otherwise, put back the bytes."
+  ;; Temporarily set the port encoding to ISO-8859-1 to allow lossless
+  ;; reading and unreading of the first line, regardless of what bytes
+  ;; are present.  Note that a valid HTTP request-line contains only
+  ;; ASCII characters.
+  (with-temporary-port-encoding socket "ISO-8859-1"
+    (lambda ()
+      ;; Save the port 'line' and 'column' counters and later restore
+      ;; them, since unreading what we read is not sufficient to do so.
+      (with-saved-port-line+column socket
+        (lambda ()
+          ;; Read up to (but not including) the first CR or LF.
+          ;; Although HTTP mandates CRLF line endings, we are permissive
+          ;; here to guard against the possibility that in some
+          ;; environments CRLF might be converted to LF before it
+          ;; reaches us.
+          (match (read-delimited "\r\n" socket 'peek)
+            ((? eof-object?)
+             ;; We found EOF before any input.  Nothing to do.
+             'done)
+
+            ((? permissive-http-request-line? request-line)
+             ;; The input from the socket began with a plausible HTTP
+             ;; request-line, which is unlikely to be legitimate and may
+             ;; indicate an possible break-in attempt.
+
+             ;; First, set the current port parameters to a void-port,
+             ;; to avoid sending any more data over the socket, to cause
+             ;; the REPL reader to see EOF, and to swallow any remaining
+             ;; output gracefully.
+             (let ((void-port (%make-void-port "rw")))
+               (current-input-port   void-port)
+               (current-output-port  void-port)
+               (current-error-port   void-port)
+               (current-warning-port void-port))
+
+             ;; Read from the socket until we would block,
+             ;; and then close it.
+             (let ((drained-input (drain-input-and-close socket)))
+
+               ;; Print a report to STDERR (POSIX file descriptor 2).
+               ;; XXX Can we do better here?
+               (call-with-port (dup->port 2 "w")
+                 (cut format <> "
+@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+@@ POSSIBLE BREAK-IN ATTEMPT ON THE REPL SERVER                @@
+@@ BY AN HTTP INTER-PROTOCOL EXPLOITATION ATTACK.  See:        @@
+@@ <https://en.wikipedia.org/wiki/Inter-protocol_exploitation> @@
+@@ Possible HTTP request received: ~S
+@@ The associated socket has been closed.                      @@
+@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n"
+                      (string-append request-line
+                                     drained-input)))))
+
+            (start-line
+             ;; The HTTP request-line was not found, so
+             ;; 'unread' the characters that we have read.
+             (unread-string start-line socket))))))))
+
+(define (guard-against-http-request socket)
+  "Arrange for the Guile REPL to check for an HTTP request in the
+initial input from SOCKET, in which case the socket will be closed.
+This guards against HTTP inter-protocol exploitation attacks, a scenario
+whereby an attacker can, via an HTML page, cause a web browser to send
+data to TCP servers listening on a loopback interface or private
+network."
+  (%set-port-property! socket 'guard-against-http-request? #t))
+
+(define* (maybe-check-for-http-request
+          #:optional (socket (current-input-port)))
+  "Apply check-for-http-request to SOCKET if previously requested by
+guard-against-http-request.  This procedure is intended to be added to
+before-read-hook."
+  (when (%port-property socket 'guard-against-http-request?)
+    (check-for-http-request socket)
+    (unless (port-closed? socket)
+      (%set-port-property! socket 'guard-against-http-request? #f))))
+
+;; Install the hook.
+(add-hook! before-read-hook
+           maybe-check-for-http-request)
+
+;;; Local Variables:
+;;; eval: (put 'with-temporary-port-encoding 'scheme-indent-function 2)
+;;; eval: (put 'with-saved-port-line+column  'scheme-indent-function 1)
+;;; End: