Re: About the 'su' command

["Karsten M. Self" <kmself at ix dot netcom dot com>]

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 (cygwin@docbill.net) 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 "bar.sh" < /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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-- 
Karsten M. Self <kmself@ix.netcom.com>        http://kmself.home.netcom.com/
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