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Re: About the 'su' command

It is.  See <>.

On Mon, 30 Jun 2003, Karsten M. Self wrote:

> Is this, or could this be made, part of the standard Cygwin docs and/or
> FAQ?
> Very nice explanation, Bill.
> Peace.
> on Wed, Jun 18, 2003 at 08:51:24AM -0400, Bill C. Riemers ( wrote:
> >
> > > The second says the command wont work unless I have appropriate
> > > privileges.
> > > Do you know "someone" on an XP station that has more powers than the
> > > Administrator or an Administrators member ?
> >
> > On most Unix systems, if you create a user with UID 65535 you will find that
> > user is unable to run 'suid' commands including 'su'.  This is result of
> > 65535 mapping to -1 as a short, and -1 having special meaning.  For awhile
> > there was a trend to make the "nobody" user 65535.  But then with the dawn
> > of the web, programmers started wanting to make SUID cgi-bin scripts, while
> > still using "nobody" as the default user for web connections.  As such, the
> > practice using 65535 for "nobody" has for the most part been abandoned in
> > the Unix world.
> >
> > However, someone at Microsoft must have thought this was an extremely good
> > idea.  And why just have one account which is not allowed to SUID?  So
> > instead, Microsoft wrote XP so any account != UID 18 is prohibited from
> > SUID.  (OK.  I over simplified, you can actually grant other accounts
> > privilege to SUID on XP professional...)
> >
> > At first thought, the idea of restricting SUID to SYSTEM seems to give XP
> > much stronger security than most unix systems.  Until, you stop and
> > consider, if only SYSTEM can SUID, and I can't login as SYSTEM, how does
> > anything ever get installed to run under SYSTEM?  It turns out SYSTEM is the
> > account used for running services.  Anyone with Administrators privilege can
> > add a new service.  Consequently, all Administrators can run any program
> > they like as SYSTEM, including of course 'su'.
> >
> > So, you ask, if it is so easy for Administrator to run a process as SYSTEM,
> > why doesn't 'su' use this trick?  Quite simple.  You can not change an
> > existing process to SYSTEM privileges, nor can you do a direct exec() so you
> > can pass your open file descriptors and environment to the new process.
> > Consequently, you would find that if su used this "trick" your process would
> > be running under a new TTY without access to existing file descriptors.  So
> > a command like, 'su root -c "" < /tmp/foo' would not work as expected.
> >
> > Now you ask, "Well then, why can ssh do pipes."  Very simple, 'ssh' sticks
> > around after starting the child process starts passing data from open file
> > descriptors though sockets.
> >
> > Finally you ask, "If ssh can do that, why doesn't su?"  Simple.  Why rewrite
> > 'su' to do those types of tricks, when 'ssh' already exists?
> >                                              Bill

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