Date: Tue, 18 Apr 2000 14:17:05 +0200
From: "RUS-CERT, University of Stuttgart" <[email protected]>
To: [email protected]Subject: RUS-CERT Advisory 200004-01: GNU Emacs 20
____________________________________________________________
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 <[email protected]>,
+ 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 <[email protected]>,
+ 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 <[email protected]>,
+ 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
