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 10:45:05 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alex Thorlton <alex.thorlton@...cle.com>
Cc:     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 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://lkml.kernel.org/r/20220303112825.068773913@infradead.org

It should probably go ahead and be merged now...

-- 
Josh