Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

From: Andy Lutomirski
Date: Mon Jun 17 2019 - 13:30:40 EST

On Mon, Jun 17, 2019 at 7:42 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Jun 12, 2019 at 07:44:12PM +0000, Nadav Amit wrote:
> > I have run into similar problems before.
> >
> > I had two problematic scenarios. In the first case, I had a âcallâ in the
> > middle of the patched code-block, but this call was always followed by a
> > âjumpâ to the end of the potentially patched code-block, so I did not have
> > the problem.
> >
> > In the second case, I had an indirect call (which is shorter than a direct
> Longer, 6 bytes vs 5 if I'm not mistaken.
> > call) being patched into a direct call. In this case, I preceded the
> > indirect call with NOPs so indeed the indirect call was at the end of the
> > patched block.
> >
> > In certain cases, if a shorter instruction should be potentially patched
> > into a longer one, the shorter one can be preceded by some prefixes. If
> > there are multiple REX prefixes, for instance, the CPU only uses the last
> > one, IIRC. This can allow to avoid synchronize_sched() when patching a
> > single instruction into another instruction with a different length.
> >
> > Not sure how helpful this information is, but sharing - just in case.
> I think we can patch multiple instructions provided:
> - all but one instruction are a NOP,
> - there are no branch targets inside the range.
> By poking INT3 at every instruction in the range and then doing the
> machine wide IPI+SYNC, we'll trap every CPU that is in-side the range.

How do you know you'll trap them? You need to IPI, serialize, and get
them to execute an instruction. If the CPU is in an interrupt and RIP
just happens to be pointed to the INT3, you need them to execute a
whole lot more than just one instruction.