Re: [PATCH 1/2] VMware detection support for x86 and x86-64

H. Peter Anvin
2008-09-25T05:31:30Z

Alok Kataria wrote:
>
> Hmm...what can a IN on an unknown port cause on native hardware, if a
> port is not being used it would return 0xFFFFFFFF in eax, and if you
> have a real device there (a sane one), what can IN result in apart from
> reading some IO register/counter value in eax ?
> If there is anything apart from the above 2 outcomes, please let me know
> exactly what you mean.
>

First, you are assuming all devices are "sane". This is obviously wrong
-- you're poking in hyperspace, and you don't know if you're going to
hit someone's ancient controller card that perhaps drives a medical
accelerator for all you know.

Second, you are assuming that devices you call "sane" don't have I/O
ports with read side effects. Many, if not most, devices have some I/O
ports with read side effects, especially read-clear semantics and/or
queue drain operations.

Third, in the real world hardware is buggy. Not just a little, but
severely so. Accessing a part of a device which is uninitialized,
powered down or plain broken can wedge the device or the whole system.

In short, poking at I/O ports which you don't know what they are at best
takes us bad to the bad old days of ISA probing (without the protection
of customary address assignments); I think it has to be an absolutely
last resort and would be reflective of utterly incompetent design. It
is significantly *worse* than stealing random opcodes, Virtual PC-style,
and that is also unacceptable.

-hpa