linux-kernel - Re: [PATCH v4 15/16] module: Move where we mark modules RO,X

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Date:   Thu, 24 Oct 2019 15:16:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jeyu@...nel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v4 15/16] module: Move where we mark modules RO,X

On Wed, Oct 23, 2019 at 12:00:25PM -0500, Josh Poimboeuf wrote:

> > This then raises a number of questions:
> > 
> >  1) why is that RELA (that obviously does not depend on any module)
> >     applied so late?
> 
> Good question.  The 'pv_ops' symbol is exported by the core kernel, so I
> can't see any reason why we'd need to apply that rela late.  In theory,
> kpatch-build isn't supposed to convert that to a klp rela.  Maybe
> something went wrong in the patch creation code.
> 
> I'm also questioning why we even need to apply the parainstructions
> section late.  Maybe we can remove that apply_paravirt() call
> altogether, along with .klp.arch.parainstruction sections.
> 
> I'll need to look into it...

Right, that really should be able to run early. Esp. after commit

  11e86dc7f274 ("x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()")

paravirt patching is unconditional. We _never_ run with the indirect
call except very early boot, but modules should have them patched way
before their init section runs.

We rely on this for spectre-v2 and friends.

> >  3) Is there ever a possible module-dependent RELA to a paravirt /
> >     alternative site?
> 
> Good question...

> > Then for 3) we only have alternatives left, and I _think_ it unlikely to
> > be the case, but I'll have to have a hard look at that.
> 
> I'm not sure about alternatives, but maybe we can enforce such
> limitations with tooling and/or kernel checks.

Right, so on IRC you implied you might have some additional details on
how alternatives were affected; did you manage to dig that up?