[Bug libc/11397] New: calls to cuserid() can result in buffer overflows

["jgeisler at cse dot taylor dot edu" <sourceware-bugzilla@sourceware.org>]

Since cuserid() uses __getpwuid_r() to obtain the user name and then strncpy()
to copy the user name to the final buffer, the buffer could end up with a
non-NUL terminated value if the login name length >= L_cuserid.  This seems
wrong since L_cuserid is defined as 9 (8 default characters + 1 NUL) for the old
setting when all usernames never exceeded 8 characters.  It seems to me that
since strncpy() is being used, we don't worry about maintaining the final result
when it is too long to fit in a buffer of L_cuserid characters.  Therefore, we
should be safe and free to cut off one additional character to ensure the buffer
truly is NUL-terminated.

As far as I can determine, this bug has existed in glibc for an extremely long
time since this interface is deprecated and probably not actively examined for
this kind of issue.  I only found it due to a legacy app that was crashing on me
when I was logged in with long usernames, but not short usernames.

My suggested patch for sysdeps/posix/cuserid.c is as follows:

--- cuserid.c   2001-12-14 02:00:48.000000000 -0500
+++ cuserid.c.new       2010-03-17 23:08:38.000000000 -0400
@@ -44,5 +44,6 @@
 
   if (s == NULL)
     s = name;
-  return strncpy (s, pwptr->pw_name, L_cuserid);
+  s[L_cuserid - 1] = '\0';
+  return strncpy (s, pwptr->pw_name, L_cuserid - 1);
 }

-- 
           Summary: calls to cuserid() can result in buffer overflows
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: critical
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: jgeisler at cse dot taylor dot edu
                CC: glibc-bugs at sources dot redhat dot com,jgeisler at cse
                    dot taylor dot edu


http://sourceware.org/bugzilla/show_bug.cgi?id=11397

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