Re: [Fwd: Truth about ssh 1.2.27 vulnerabiltiy]

[long () KESTREL CC UKANS EDU (Jeff Long)]

Oh crud, this wasn't tested on Digital Unix 4.0D.  It was tested before
and after on Compaq Tru64 5.0.

Jeff Long

Jeff Long wrote:
> Dan Astoorian wrote:
> > On Wed, 29 Sep 1999 16:59:48 EDT, Sylvain Robitaille writes:
> > > I don't promise the most impressive code, but it has been tested (on
> > > Digital Unix) and I believe it works correctly. Comments are of course
> > > welcome...
> >
> > I have a couple of serious concerns about this patch.
> >
> > 1) It leaves behind a race condition; a symlink created between the
> >    lstat() and the bind() will still get happily followed.  The race
> >    condition could be minimized by moving the lstat() and the bind()
> >    closer together, but it can't be eliminated this way.  This is why
> >    it's important for the check to be made in the kernel, where it can
> >    be done atomically.
> <snip>
> > The race condition is a hard problem; if bind() follows symlinks, it is
> > *impossible* to safely use it in a directory writable by anyone other
> > than geteuid().
> >
> > I haven't looked into what would be involved in creating a proper patch,
> > but appropriate ways to fix the problem *might* include:
> >
> > - changing the process's effective userid/groupid/credentials to match
> >   the target user before doing the bind(), so that directories not
> >   writable by the user are also not writable by the code doing the
> >   bind(); or
> <snip>
>
> Seeing the race problems with the previous two patches I thought I would
> take a shot at one.  It changes the effective uid/gid to the user
> logging in before doing the bind() (and then resets them after) which
> seems to take care of the problem.  It also chown's the
> /tmp/ssh-<username> directory before doing the bind in the case that it
> did not already exist so that the user owns it before performing the
> bind.  On Digital Unix 4.0D this seems to take care of the problem.  The
> bind() will fail if a symlink exists to a file that the user would
> normally not be able to write to (such as /etc/nologin).  The only other
> difference after logging in is that the socket will now be owned by the
> user instead of root but I can't think of a reason why this would be
> bad.
>
> If anyone sees problems in this patch please let me know.
>
> Jeff Long

*** newchannels.c.original      Thu Sep 30 16:58:22 1999
--- newchannels.c       Thu Sep 30 17:13:24 1999
***************
*** 2262,2267 ****
--- 2262,2269 ----
    struct stat st, st2, parent_st;
    mode_t old_umask;
    char *last_dir;
+   uid_t saved_euid = 0;
+   gid_t saved_egid = 0;

    if (auth_get_socket_name() != NULL)
      fatal("Protocol error: authentication forwarding requested twice.");
***************
*** 2411,2427 ****
       creating unix-domain sockets, you might not be able to use
       ssh-agent connections on your system */
    old_umask = umask(S_IRUSR|S_IXUSR|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH);
!
!   if (bind(sock, (struct sockaddr *)&sunaddr, AF_UNIX_SIZE(sunaddr)) < 0)
!     packet_disconnect("Agent socket bind failed: %.100s", strerror(errno));
!
!   umask(old_umask);
!
    if (directory_created)
      if (chown(".", pw->pw_uid, pw->pw_gid) < 0)
        packet_disconnect("Agent socket directory chown failed: %.100s",
                          strerror(errno));

    /* Start listening on the socket. */
    if (listen(sock, 5) < 0)
      packet_disconnect("Agent socket listen failed: %.100s", strerror(errno));
--- 2413,2442 ----
       creating unix-domain sockets, you might not be able to use
       ssh-agent connections on your system */
    old_umask = umask(S_IRUSR|S_IXUSR|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH);
!
    if (directory_created)
      if (chown(".", pw->pw_uid, pw->pw_gid) < 0)
        packet_disconnect("Agent socket directory chown failed: %.100s",
                          strerror(errno));

+   saved_euid = geteuid();
+   saved_egid = getegid();
+
+   if (setegid(pw->pw_gid) < 0)
+       packet_disconnect("Agent socket setegid failed: %.100s", strerror(errno));
+   if (seteuid(pw->pw_uid) < 0)
+       packet_disconnect("Agent socket seteuid failed: %.100s", strerror(errno));
+
+   if (bind(sock, (struct sockaddr *)&sunaddr, AF_UNIX_SIZE(sunaddr)) < 0)
+     packet_disconnect("Agent socket bind failed: %.100s", strerror(errno));
+
+   if (seteuid(saved_euid) < 0)
+       packet_disconnect("Agent socket re-seteuid failed: %.100s", strerror(errno));
+   if (setegid(saved_egid) < 0)
+       packet_disconnect("Agent socket re-setegid failed: %.100s", strerror(errno));
+
+   umask(old_umask);
+
    /* Start listening on the socket. */
    if (listen(sock, 5) < 0)
      packet_disconnect("Agent socket listen failed: %.100s", strerror(errno));