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Groups > alt.os.development > #9213

Re: Unlocking writes to VGA ROM (C0000)

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On Wed, 24 Feb 2016 13:06:11 +0000
James Harris <james.harris.1@gmail.com> wrote:

> On 24/02/2016 08:55, dawid.ciecierski@gmail.com wrote:
> > Hello!
> >
> > As this is my first post let me first say thank you to everyone
> > involved in these conversations. I'm impressed such a group exists
> > and thankful you spend your free (and not free) time sharing the
> > often very technical things you know.  
> 
> Welcome to the group! And thanks for posting an answer to your
> question in case it helps others - such as me if I ever get to do the
> same thing.

James,

What do you think about being able to "enable write access
to the VGA ROM memory area" via MTRRs even though the
"[integrated GPU] does not utilize this area"?  I.e., this
area is normally write protected, historically, on x86
platforms.  The last architecture where I recall that this
ability to unmap ROM regions was available was the old
Commodore C64 (and probably others by Commodore).  Should
one be able to unlock the region via MTRRs if not in use?
I.e., could this be used for an injection attack via a
BIOS call to an inserted fake VGA ROM BIOS?  IMO, it's
unlikely that much OS software has enabled protection
against calling the BIOS routines, expecting them to be
safe, including the VGA region. There might even be some
software which tests the region and expects to find it
non-writable which would fail.


Rod Pemberton

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Unlocking writes to VGA ROM (C0000) dawid.ciecierski@gmail.com - 2016-02-24 00:55 -0800
  Re: Unlocking writes to VGA ROM (C0000) dawid.ciecierski@gmail.com - 2016-02-24 03:38 -0800
  Re: Unlocking writes to VGA ROM (C0000) James Harris <james.harris.1@gmail.com> - 2016-02-24 13:06 +0000
    Re: Unlocking writes to VGA ROM (C0000) Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-02-24 18:05 -0500
      Re: Unlocking writes to VGA ROM (C0000) "wolfgang kern" <nowhere@never.at> - 2016-02-25 11:53 +0100
      Re: Unlocking writes to VGA ROM (C0000) dawid.ciecierski@gmail.com - 2016-02-25 03:34 -0800
      Re: Unlocking writes to VGA ROM (C0000) James Harris <james.harris.1@gmail.com> - 2016-02-25 15:04 +0000

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