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From: Gary Clark II <gclarkii@main.gbdata.com>
Message-Id: <199610291105.FAA13009@main.gbdata.com>
Subject: Re: Why is PSL_VM excluded from USERCHANGE macro?
To: johnson@charming.nrtc.northrop.com
Date: Tue, 29 Oct 1996 05:05:44 -0600 (CST)
Cc: questions@freebsd.org
In-Reply-To: <9610282237.AA07533@charming.nrtc.northrop.com> from "johnson@charming.nrtc.northrop.com" at "Oct 28, 96 02:37:49 pm"
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johnson@charming.nrtc.northrop.com wrote:
> 
> 
> My question:  Why is PSL_VM excluded from the set that the user can
> modify?  (If PSL_VM is set in EFLAG, the processor starts executing in
> Virtual 8086 mode.)
> 
> What would the dangers be to system integrity if a user process started
> executing in Virtual 8086 mode?
Because to run in VM86 mode you have to do a fair of amount of setup.
If this is not done, nasty things can happen.

> 
> While the processor is executing in Virtual 8086 mode, the paging hardware
> still is enabled.  It would appear that a user process operating in Virtual
> 8086 mode could do no harm to the kernel data structures or otherwise
> compromise the security of the machine.  It might be useful to permit processes
> to operate in Virtual 8086 mode, for purposes of creating an operating
> environment to support execution of old 8086 code.
It requires ALOT more than just running in VM86 mode.  I wish it was that
easy:)

> 
> 
> Thanks,
> 
> Greg Johnson
> johnson@nrtc.northrop.com
> 
Gary

-- 
Gary Clark II   (N5VMF) |    I speak only for myself and "maybe" my company 
gclarkii@GBData.COM     |          Member of the FreeBSD Doc Team 
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