PIQ + Pentium, + some general protection thoughts for the HCU

The PIQ has been used for a long time to detect whether a prog is being
traced. It is a simple idea, but actually determining the _size_ of the
PIQ can be another matter altogether. The two PIQs on the Pentium are
both 64 bytes long (though only one is in use at any one time) so you
don’t need to work it out. The 486’s is 32 and the 386’s 16 (though
some versions have 12).

First though, +Heres was right in suspecting that the Pentium always
updates the PIQ. At first, I couldn’t believe that this was actually the
way it’s done, but according to a book I’ve got it is in fact true. The
Pentium checks the linear address of the memory address to be modified
to see if it’s in the PIQ. I’m not 100% sure, but it appears that even
instructions in the pipeline are also saved. (When you get to execute
the ‘rep stosb’ there should be two ‘inc bx’s in the pipeline).

I’m not entirely sure whether this feature is a good idea in terms of
CPU performance. I’d have to see some evidence before deciding.
Personally, I wouldn’t have thought that doing a check on every memory
write just to stop self-modifying code was worth it.

However, all is not lost! You _can_ use the PIQ for this trick, but only
in protected mode. Consider the linear address of the instruction to
modify and then consider the linear address of that same instruction in
the PIQ. They’re the same, right? Well, they are and they aren’t. If you
remap memory so you’ve got virtual memory you could have a different
linear address that is in fact mapped to the same physical memory
address. Hence if you’re running normally you’ll get what was in the
PIQ, but if there’s a debugger running then the PIQ will be different
and you’ll get what’s in memory. Simple really 🙂


This different way of doing things on the Pentium is actually pretty
beneficial. As most readers will have noticed, the main problem with
this type of code is that it’s really easy to spot, but on the Pentium
you get code like:

    set address $12345678 to 10      ; these would in fact map to the
    set address $87654321 to 0       ; same address

(NB: I assume that $12345678 is the linear address to modify – if it
isn’t then you’ll have to do the modification before the address to be
modified gets into the pipeline. If you don’t the memory address will be
altered, but not the bit in the pipeline. Of course, you can have the
instruction be something like ‘mov eax,10’ already but more useful is
to change the instructions to what the real ones are, or something else
entirely if a trace is on as this will fool disassemblers)

If you’re sneaky (which you should be) you can exploit the Pentium’s
large PIQ to full extent – you could have two instructions like this
approx 60 bytes apart. Only if you’re not being traced will the value
be equal to 10. You can exploit this virtual memory mapping even more
by using different addresses to read the location later on. Clearly,
the location could be used as an instruction or a variable, or even
both at the same time. Obviously, when setting up the mapping you’ll
have to be sneaky 🙂 To make things more difficult you could hide
instructions within instructions and put the byte sequence F0 0F C7
C8 in somewhere as it puts Pentium’s into an endless loop due to a bug.

A neat trick is to have the protection routine write itself using a
stealth technique to provide different length routines and have the
protection routine then write vital bits of the code if the protection
is passed. The writing routine itself could use the PIQ trick and so
could the protection. Your protection routine should be the longest
most convoluted spaghettified piece of garbage you can come up with
and contain lots of jumps, checks and as much bizarre code as you can
fit in, possibly with lots of stuff that’s there simply ‘to make the
numbers up.’ You should, of course, have repeated protection checks,
at different points in the program, that jump into the protection
code at different points. You can even have multiple protections.
Also, don’t just blindly take the address and stick it in your code,
build it elsewhere using several instructions, preferably at
different code points.

If the Trap Flag is set, a trap occurs after each instruction. If this
is being used by the debugger then you can use the Pentium’s internal
counters to verify this. Use RDTSC to get the clock count and then read
it later on. You will already know a rough number of how long your
code takes so you can check this to see if you’re being traced. I’ve no
idea how SoftIce or other debuggers work. This is probably one way.
In all cases of having the protection being broken in any way I
recommend crashing the machine (or doing something equally obtuse such
as re-writing bits of the code). Obviously, there are a wide variety
of ways of doing this and I leave the choice to the reader. There’s
no reason in my mind why you should make things easy. As an aside,
if you set the Resume Flag, this disables Int 1 which is used for
debugging under protected mode, or you could patch in your own
version instead.

A final trick is to do a file check, e.g. file length or bits of the
file to see if its been changed, or the actual code in memory. If it
has you could freeze the program somewhere or erase all the code from
memory except a single infinite loop or delete the proggy from the disk
alongwith its associated files (be careful not to delete anything you
shouldn’t though) or…you get the idea I guess. A final point to note
is that I think you should always use multiple protections and should
make life difficult by doing clearing the proggy from memory and stuff
like that. It’s better than nothing I suppose.

I must stress that I’m an absolute newbie to PC cracking/hacking and
have very little experience in the field itself. Hopefully the info
here is correct and I’ve not written anything totally lame but if I
have then please correct me and don’t flame too hard. If you’re going
to be doing any PIQ stuff then I suggest you read up on exactly how it
works, as it is quite complicated, but there should be some ideas in
here for you to work on.

As a final point, I think that only prgs should be available for
download first, with sources following later, and there should be no
help given. I mean, how many times have you bought a prg that comes
with source or has hints on how to crack it?


Kaynak: www.woodmann.com/fravia

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