linux-kernel - Re: [PATCH] x86/paravirt: Apply paravirt instructions in consistent order during boot/module load

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Date:   Mon, 7 Mar 2022 13:02:17 -0600
From:   Alex Thorlton <alex.thorlton@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Alex Thorlton <alex.thorlton@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/paravirt: Apply paravirt instructions in consistent
 order during boot/module load

On Mon, Mar 07, 2022 at 10:45:05AM -0800, Josh Poimboeuf wrote:
> On Mon, Mar 07, 2022 at 12:03:38PM -0600, Alex Thorlton wrote:
> > Commit 4e6292114c74 ("x86/paravirt: Add new features for paravirt
> > patching") changed the order in which altinstructions and paravirt
> > instructions are patched at boot time.  However, no analogous change was
> > made in module_finalize, where we apply altinstructions and
> > parainstructions during module load.
> > 
> > As a result, any code that generates "stacked up" altinstructions and
> > parainstructions (i.e. local_irq_save/restore) will produce different
> > results when used in built-in kernel code vs. kernel modules.  This also
> > makes it possible to inadvertently replace altinstructions in the booted
> > kernel with their parainstruction counterparts when using
> > livepatch/kpatch.
> > 
> > To fix this, re-order the processing in module_finalize, so that we do
> > things in this order:
> > 
> >  1. apply_paravirt
> >  2. apply_retpolines
> >  3. apply_alternatives
> >  4. alternatives_smp_module_add
> > 
> > This is the same ordering that is used at boot time in
> > alternative_instructions.
> > 
> > Fixes: 4e6292114c74 ("x86/paravirt: Add new features for paravirt patching")
> > Signed-off-by: Alex Thorlton <alex.thorlton@...cle.com>
> > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> 
> Peter previously posted a fix, buried in his IBT series:
> 
>   https://urldefense.com/v3/__https://lkml.kernel.org/r/20220303112825.068773913@infradead.org__;!!ACWV5N9M2RV99hQ!YARvXhahbleGAt689pqTXJU7ko-rePIjzrbuGmemJXgFRViFZ8FDfOy7mHZQ7CPaG6Y$ 
> 
> It should probably go ahead and be merged now...

Ahh, yep - hadn't seen that one yet!  In any case, I'm glad this is on
other folk's radar.

Thanks for letting me know, Josh!

- Alex