____________________________________________________________

                RUS-CERT Advisory 200004-01: GNU Emacs 20

RUS-CERT, University of Stuttgart

2000-04-18

Summary

   Several vulnerabilities were discovered in all Emacs versions up
   to 20.6, namely:

     Under certain circumstances, unprivileged local users can
          eavesdrop the communication between Emacs and its
          subprocesses.

     It is impossible to safely create temporary files in a public
          directory from Emacs Lisp.

     The history of recently typed keys may expose passwords.

   Especially the first two vulnerabilities seriously impact the use
   of tools like Mailcrypt in a multi-user environment.

1. Improper permissions on slave PTYs

  1.1. Scope

   Affected systems:

     Linux (both libc and glibc)

     FreeBSD (and probably other *BSD variants)

     HP-UX 10.x, 11.00

     AIX 4

   Unaffected systems:

     Solaris (The Solaris runtime system automatically adjusts the
          PTY permissions.)

   Data General's DG/UX seems to be unaffected, according to the
   source code. Other systems have not been examined.

  1.2. Severity

   High in multi-user environments, low otherwise.

  1.3. Problem

   On the systems listed above, when a new subprocess is created
   using the builtin Lisp function start-process, Emacs doesn't set
   proper permissions for the slave PTY device.

  1.4. Impact

   Unprivileged local users can eavesdrop the data which Emacs sends
   to its subprocess and fake responses from the subprocess. This
   impacts Emacs packages such as Mailcrypt, which transmit (among
   other things) PGP passphrases over this data channel.

  1.5. Solution

   At Emacs Lisp level, the only workaround is to use call-process
   instead of start-process. Of course, this is not always an option
   because the functionality provided by these functions is not the
   same (synchronous vs. asynchronous subprocesses).

   The real solution requires modification of the Emacs C source
   code. A patch for Emacs 20.6 is included below which enables
   Emacs to Unix98 PTYs. The patch is known to work on the following
   systems:

     Linux with glibc 2.1

     AIX 4.2

     HP-UX 11.00

   It is expected to work on HP-UX 10.x as well. (Under some
   versions of HP-UX, grantpt() does not behave as specified. The
   patch contains a suitable workaround.)

   Unfortunately, systems lacking Unix98 support (such as Linux with
   libc5 and glibc 2.0, FreeBSD and AIX 3) require a completely
   different fix and a setuid root binary to change the PTY
   permissions (in other words: some kind of userspace Unix 98 PTY
   emulation). There are no plans to provide this emulation; Unix 98
   PTYs are already widely adopted and most Unix derivatives provide
   them (with the notable exception of several *BSD variants). For
   FreeBSD, an enhancement to openpty() has been proposed which sets
   proper permissions on the slave TTY device (see problem report
   bin/9770). The proposal has yet to be adopted, though.

   Future Emacs releases will contain a similar fix.

2. Unsafe creation of temporary files

  2.1. Scope

   All Unix-like Emacs platforms on which public directories are
   used to store temporary files.

  2.2. Severity

   High in multi-user environments, low otherwise.

  2.3. Problem

   Emacs Lisp does not provide any functionality to create a file in
   a publicly writable directory in a safe way.

  2.4. Impact

   Many Emacs packages use the make-temp-name Lisp function to
   create names for temporary files. These names are not very hard
   to guess. Because it is impossible to create the actual temporary
   file in a safe manner, the usual symlink attacks are likely
   successful.

  2.5. Solution

   Emacs 21 will provide a new make-temp-file function (which
   creates the file in question in safe way) and the functionality
   to safely create temporary files. In the meantime, until Emacs 21
   is released and package maintainers adopt the new function,
   private directories for temporary files should be used. Most
   packages provide variables for that. For example, for Mailcrypt,
   the variable mc-temp-directory has to be set, and for Python
   Mode, it's py-temp-directory.

3. Passwords are stored in the key history

  3.1. Scope

   All platforms.

  3.2. Severity

   Low.

  3.3. Problem

   Functions like read-passwd do not clear the the history of
   recently typed keys. In fact, there is no way to do that from
   Emacs Lisp.

  3.4. Impact

   Passwords might be recovered by someone who has got access to the
   console on which Emacs is running, subverting password expiring
   as, for example, provided by Mailcrypt. (Usually, there are many
   other ways to obtain passwords if you can type C-h l inside a
   foreign Emacs, though.)

  3.5. Solution

   The patch below adds code to clear-this-command-keys which will
   erase the vector containing the last 100 events. In the past,
   this function was already used as if it behaved that way.

4. Acknowledgements

   Helmut Waitzmann for rediscovering the PTY permissions problem
   and testing the HP-UX patch. Gerd Moellmann of the Emacs
   development team for the patch to clear-this-command-keys and
   helpful comments.

5. Patch against Emacs 20.6

   The patch below is against GNU Emacs 20.6, as available at GNU
   FTP mirrors. Note that you have to run autoconf to recreate the
   configure script (including it would have enormously increased
   the size of the patch).


diff --unified --recursive emacs-20.6-orig/configure.in emacs-20.6/configure.in
--- emacs-20.6-orig/configure.in        Sat Feb 26 13:07:02 2000
+++ emacs-20.6/configure.in     Fri Mar 10 19:13:05 2000
@@ -1636,6 +1636,11 @@
 strerror fpathconf select mktime euidaccess getpagesize tzset setlocale \
 utimes setrlimit setpgid getcwd shutdown strftime)

+# Check for UNIX98 PTYs.
+# getpt is a glibc addition which emulates the master device on
+# systems without kernel support.
+AC_CHECK_FUNCS(grantpt unlockpt getpt ptsname)
+
 # Check this now, so that we will NOT find the above functions in ncurses.
 # That is because we have not set up to link ncurses in lib-src.
 # It's better to believe a function is not available
diff --unified --recursive emacs-20.6-orig/src/config.in emacs-20.6/src/config.in
--- emacs-20.6-orig/src/config.in       Mon Apr 26 07:19:44 1999
+++ emacs-20.6/src/config.in    Fri Mar 10 19:13:05 2000
@@ -235,6 +235,14 @@
 #undef HAVE_SHUTDOWN
 #undef HAVE_STRFTIME

+/* UNIX98 PTY support functions
+   getpt is a glibc addition which emulates the master device on
+   systems without kernel support. */
+#undef HAVE_GRANTPT
+#undef HAVE_UNLOCKPT
+#undef HAVE_GETPT
+#undef HAVE_PTSNAME
+
 #undef LOCALTIME_CACHE
 #undef HAVE_INET_SOCKETS

diff --unified --recursive emacs-20.6-orig/src/keyboard.c emacs-20.6/src/keyboard.c
--- emacs-20.6-orig/src/keyboard.c      Thu Nov 18 05:57:32 1999
+++ emacs-20.6/src/keyboard.c   Fri Mar 10 19:13:05 2000
@@ -8318,10 +8318,18 @@

 DEFUN ("clear-this-command-keys", Fclear_this_command_keys,
   Sclear_this_command_keys, 0, 0, 0,
-  "Clear out the vector that `this-command-keys' returns.")
+  "Clear out the vector that `this-command-keys' returns.\n\
+Clear vector containing last 100 events.")
   ()
 {
+  int i;
+
   this_command_key_count = 0;
+
+  for (i = 0; i < XVECTOR (recent_keys)->size; ++i)
+    XVECTOR (recent_keys)->contents[i] = Qnil;
+  total_keys = 0;
+  recent_keys_index = 0;
   return Qnil;
 }

diff --unified --recursive emacs-20.6-orig/src/s/aix4.h emacs-20.6/src/s/aix4.h
--- emacs-20.6-orig/src/s/aix4.h        Sat Jul 25 08:45:27 1998
+++ emacs-20.6/src/s/aix4.h     Fri Mar 17 20:44:08 2000
@@ -12,3 +12,33 @@
 /* Specify the type that the 3rd arg of `accept' points to.
    It is just a guess which versions of AIX need this definition.  */
 #define SOCKLEN_TYPE int
+
+#if defined(HAVE_GRANTPT) && defined(HAVE_UNLOCKPT) && defined(HAVE_PTSNAME)
+/* UNIX98 PTYs are available.
+   Added by Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>,
+   RUS-CERT, University of Stuttgart.  Based on Emacs code for DGUX. */
+
+/* Most of the #defines are already provided by aix3-1.h. */
+
+/* This sets the name of the slave side of the PTY.  grantpt(3) and
+   unlockpt(3) may fork a subprocess, so keep sigchld_handler() from
+   intercepting that death. */
+
+#undef  PTY_TTY_NAME_SPRINTF
+#define PTY_TTY_NAME_SPRINTF                   \
+  {                                            \
+    char *ptsname(), *ptyname;                 \
+                                               \
+    sigblock(sigmask(SIGCHLD));                        \
+    if (grantpt(fd) == -1)                     \
+      fatal("could not grant slave pty");      \
+    if (unlockpt(fd) == -1)                    \
+      fatal("could not unlock slave pty");     \
+    sigunblock(sigmask(SIGCHLD));              \
+    if (!(ptyname = ptsname(fd)))              \
+      fatal ("could not enable slave pty");    \
+    strncpy(pty_name, ptyname, sizeof(pty_name)); \
+    pty_name[sizeof(pty_name) - 1] = 0;                \
+  }
+
+#endif
diff --unified --recursive emacs-20.6-orig/src/s/gnu-linux.h emacs-20.6/src/s/gnu-linux.h
--- emacs-20.6-orig/src/s/gnu-linux.h   Wed Jan 26 14:28:40 2000
+++ emacs-20.6/src/s/gnu-linux.h        Fri Mar 17 20:44:31 2000
@@ -307,3 +307,49 @@
 #ifdef DOUG_LEA_MALLOC
 #undef REL_ALLOC
 #endif
+
+#if defined(HAVE_GRANTPT) && defined(HAVE_UNLOCKPT) && defined(HAVE_PTSNAME)
+/* UNIX98 PTYs are available.
+   Added by Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>,
+   RUS-CERT, University of Stuttgart.  Based on Emacs code for DGUX. */
+
+#define PTY_ITERATION for (i = 0; i < 1; i++)
+/* no iteration at all */
+
+/* Use getpt() if it's available, because it provides Unix98 PTY
+   emulation for kernels which doesn't support it natively. */
+
+#ifdef HAVE_GETPT
+#define PTY_OPEN                                 \
+  do {                                           \
+    fd = getpt();                             \
+    if (fcntl (fd, F_SETFL, O_NDELAY) == -1)  \
+      fatal ("could not set master PTY to non-block mode"); \
+  } while (0)
+
+#else
+/* the master PTY device */
+#define PTY_NAME_SPRINTF strcpy (pty_name, "/dev/ptmx");
+#endif
+
+/* This sets the name of the slave side of the PTY.  grantpt(3) and
+   unlockpt(3) may fork a subprocess, so keep sigchld_handler() from
+   intercepting that death. */
+
+#define PTY_TTY_NAME_SPRINTF                   \
+  {                                            \
+    char *ptsname(), *ptyname;                 \
+                                               \
+    sigblock(sigmask(SIGCHLD));                        \
+    if (grantpt(fd) == -1)                     \
+      fatal("could not grant slave pty");      \
+    if (unlockpt(fd) == -1)                    \
+      fatal("could not unlock slave pty");     \
+    if (!(ptyname = ptsname(fd)))              \
+      fatal ("could not enable slave pty");    \
+    strncpy(pty_name, ptyname, sizeof(pty_name)); \
+    pty_name[sizeof(pty_name) - 1] = 0;                \
+    sigunblock(sigmask(SIGCHLD));              \
+  }
+
+#endif
diff --unified --recursive emacs-20.6-orig/src/s/hpux.h emacs-20.6/src/s/hpux.h
--- emacs-20.6-orig/src/s/hpux.h        Mon Jan 15 10:16:40 1996
+++ emacs-20.6/src/s/hpux.h     Wed Mar 29 08:40:52 2000
@@ -228,6 +228,59 @@
 /* This is needed for HPUX version 6.2; it may not be needed for 6.2.1.  */
 #define SHORT_CAST_BUG

+#if defined(HAVE_GRANTPT) && defined(HAVE_UNLOCKPT) && defined(HAVE_PTSNAME)
+/* UNIX98 PTYs are available.
+   Added by Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>,
+   RUS-CERT, University of Stuttgart.  Based on Emacs code for DGUX. */
+
+#ifdef emacs
+#include <grp.h>
+#include <sys/stropts.h>
+#endif
+
+#define PTY_ITERATION for (i = 0; i < 1; i++)
+/* no iteration at all */
+
+/* the master PTY device */
+#define PTY_NAME_SPRINTF strcpy (pty_name, "/dev/ptmx");
+
+/* This sets the name of the slave side of the PTY.  grantpt(3) and
+   unlockpt(3) may fork a subprocess, so keep sigchld_handler() from
+   intercepting that death.  grantpt() behavior on HP-UX differs from
+   what's specified in the man page: the group of the slave PTY is set
+   to the user's primary group, and we fix that.  */
+
+#define PTY_TTY_NAME_SPRINTF                   \
+  {                                            \
+    char *ptsname(), *ptyname;                 \
+    struct group *getgrnam (), *tty_group = getgrnam ("tty"); \
+    if (tty_group == NULL)                      \
+      fatal ("group tty not found");            \
+                                               \
+    sigblock(sigmask(SIGCHLD));                        \
+    if (grantpt(fd) == -1)                     \
+      fatal("could not grant slave pty");      \
+    if (!(ptyname = ptsname(fd)))              \
+      fatal ("could not enable slave pty");    \
+    strncpy(pty_name, ptyname, sizeof(pty_name)); \
+    pty_name[sizeof(pty_name) - 1] = 0;                \
+    if (chown (pty_name, (uid_t) -1, tty_group->gr_gid) == -1) \
+      fatal ("could not chown slave pty");      \
+    if (unlockpt(fd) == -1)                    \
+      fatal("could not unlock slave pty");     \
+    sigunblock(sigmask(SIGCHLD));              \
+  }
+
+/* Push various streams modules onto a PTY channel. */
+
+#define SETUP_SLAVE_PTY \
+  if (ioctl (xforkin, I_PUSH, "ptem") == -1)   \
+    fatal ("ioctl I_PUSH ptem", errno);                \
+  if (ioctl (xforkin, I_PUSH, "ldterm") == -1) \
+    fatal ("ioctl I_PUSH ldterm", errno);
+
+#else /* no UNIX98 PTYs */
+
 /* This is how to get the device name of the tty end of a pty.  */
 #define PTY_TTY_NAME_SPRINTF \
             sprintf (pty_name, "/dev/pty/tty%c%x", c, i);
@@ -235,6 +288,8 @@
 /* This is how to get the device name of the control end of a pty.  */
 #define PTY_NAME_SPRINTF \
        sprintf (pty_name, "/dev/ptym/pty%c%x", c, i);
+
+#endif /* UNIX 98 PTYs */

 /* This triggers a conditional in xfaces.c.  */
 #define XOS_NEEDS_TIME_H