Re: [Qemu-devel] QEMU, self-modifying code, and Windows 7 64-bit (no KVM)

[Hulin, Patrick - 0559 - MITLL]

On Aug 17, 2014, at 1:21 AM, Paolo Bonzini <address@hidden> wrote:

> Il 15/08/2014 23:49, Hulin, Patrick - 0559 - MITLL ha scritto:
>>>> In this case, the write is 8 bytes and unaligned, so it gets split
>>>> into 8 single-byte writes. In stock QEMU, these writes are done in
>>>> reverse order (see the loop in softmmu_template.h, line 402). The
>>>> third decryption xor from Kernel Patch Protection should hit 4 bytes
>>>> that are in the current TB and 4 bytes in the TB afterwards in linear
>>>> order. Since this happens in reverse order, and the last 4 bytes of
>>>> the write do not intersect the current TB, those writes happen
>>>> successfully and QEMU's memory is modified. The 4th byte in linear
>>>> order (the 5th in temporal order) then triggers the
>>>> current_tb_modified flag and cpu_restore_state, longjmp'ing out.
>>>> 
>>> Would it work to just call tb_invalidate_phys_page_range before the
>>> helper_ret_stb loop?
>> 
>> Maybe. I think there’s another issue, which is that QEMU’s ending up
>> in the I/O read/write code instead of the normal memory RW. This could
>> be QEMU messing up, it could be PatchGuard doing something weird, or it
>> could be me misunderstanding what’s going on. I never really figured out
>> how the control flow works here.
> 
> That's okay.  Everything that's in the slow path goes down
> io_mem_read/write (in this case TLB_NOTDIRTY is set for dirty-page
> tracking and causes QEMU to choose that path).
> 
> Try making a self-contained test case using the kvm-unit-tests harness
> (git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git).
> 
> Paolo

Okay. I’ve attached a test case. Note that since this has to run without KVM it 
messes around with the run script.

selfmodify.patch
Description: selfmodify.patch