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From: carl@CITHEX.CALTECH.EDU.UUCP
Newsgroups: mod.computers.vax
Subject: Re: Submission for mod-computers-vax
Message-ID: <870331080702.09g@CitHex.Caltech.Edu>
Date: Tue, 31-Mar-87 11:07:11 EST
Article-I.D.: CitHex.870331080702.09g
Posted: Tue Mar 31 11:07:11 1987
Date-Received: Fri, 3-Apr-87 04:17:57 EST
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 > Can some one point me to a guide on secure use of the bypass privilege?
 > I want to write a privileged program to access a file, for example
 > SYS$SYSTEM:WHATEVER.DAT.  What keeps a user from redefining SYS$SYSTEM
 > to point to some other directory and then running the program so that
 > it accesses the wrong file?

There are two ways that VMS utilities do this:
1)  They specify a logical name table  to  use  for  the  translation  of  the
    logical  name;  thus,  a  redefinition  of  the logical name in the user's
    process, job, or group logical name table wouldn't  affect  the  program's
    behavior, and the user needs SYSNAM to define a logical name in the system
    name table.
2)  They ignore user- and supervisor-mode logical names, so the  logical  name
    must  be  defined  in EXECUTIVE or KERNEL mode; to do this, the user would
    need CMKRNL priv.
Since either CMKRNL  or  SYSNAM  priv  is  sufficient  to  suborn  the  entire
operating system, these checks should be sufficient for your purposes.

 > I also want to access a file from a privileged program as if I were
 > some user other than the one running the program.  In other words,
 > user A runs the program and asks to do something that requires updating
 > file F.  The program knows this is ok if user B could do the same thing
 > to file F without any special privileges.  What does the program do to
 > determine this?

The easiest (and probably best) way to do this is to use SYS$CHECKACCESS.  You
tell the system service what type of object you want to check, the name of the
object, the username whose access you want to check, and a description of what
you  want  to  know about the way the user could access the object (e.g., what
type of access you want to check, which access modes you want to  check,  what
privs  he has that he'd need to use to access the object, what security alarms
he'd set off if he accessed it, or if an ACL granted him access, which ACE  in
the  ACL gave him access).  The system service apparently does not distinguish
between the case in which the user you're interested in doesn't have the privs
to  access  the object and the case in which you don't have the privs to check
to see if the other user has the privs necessary to access the object.