linux-kernel - Re: [PATCHv2] x86/alternatives: Disable KASAN in apply

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Date:   Wed, 11 Oct 2023 22:45:27 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        Fei Yang <fei.yang@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCHv2] x86/alternatives: Disable KASAN in apply_alternatives()


* Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:

> On Wed, Oct 11, 2023 at 09:46:16AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 11, 2023 at 09:58:49AM +0300, Kirill A. Shutemov wrote:
> > > Fei has reported that KASAN triggers during apply_alternatives() on
> > > 5-level paging machine:
> > > 
> > 
> > Urgh @ KASAN splat, can't we summarize that?
> 
> What about this?
> 
> 	BUG: KASAN: out-of-bounds in rcu_is_watching
> 	Read of size 4 at addr ff110003ee6419a0 by task swapper/0/0
> 	...
> 	__asan_load4
> 	rcu_is_watching
> 	? text_poke_early
> 	trace_hardirqs_on
> 	? __asan_load4
> 	text_poke_early
> 	apply_alternatives
> 	...
> 
> Is it enough details or I overdid summarization?

No, that's perfect IMO. I'd even leave out the unreliable '?' entries:

> 	BUG: KASAN: out-of-bounds in rcu_is_watching
> 	Read of size 4 at addr ff110003ee6419a0 by task swapper/0/0
> 	...
> 	__asan_load4
> 	rcu_is_watching
> 	trace_hardirqs_on
> 	text_poke_early
> 	apply_alternatives
> 	...

... or so.

> 	/*
> 	 * In the case CONFIG_X86_5LEVEL=y, KASAN_SHADOW_START is defined using
> 	 * cpu_feature_enabled(X86_FEATURE_LA57) and is therefore patched here.
> 	 * During the process, KASAN becomes confused seeing partial LA57
> 	 * conversion and triggers a false-positive out-of-bound report.
> 	 *
> 	 * Disable KASAN until the patching is complete.
> 	 */
> 
> Looks good?

LGTM.

Thanks,

	Ingo