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

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Date:   Fri, 25 Oct 2019 08:44:56 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, 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 Thu 2019-10-24 15:16:34, Peter Zijlstra wrote:
> 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.

Hmm, the original bug report against livepatching was actually about
paravirt ops, see below.


> > 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.

Livepatching has the same requirement. The module code has to be fully
livepatched before the module gets actually used. It means before
mod->init() is called and before the module is moved into
MODULE_STATE_LIVE state.


> > >  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?

I am not sure what Josh had in mind. But the problem with livepatches,
paravort ops, and alternatives was described in the related patchset, see
https://lkml.kernel.org/r/1471481911-5003-1-git-send-email-jeyu@redhat.com

The original bug report is
https://lkml.kernel.org/r/20160329120518.GA21252@canonical.com

Best Regards,
Petr