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

[Paolo Bonzini]

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