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Date:   Tue, 10 Oct 2023 13:24:34 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        Fei Yang <fei.yang@intel.com>, stable@vger.kernel.org
Subject: Re: [PATCH] x86/alternatives: Disable KASAN on text_poke_early() in
 apply_alternatives()
Message-ID: <20231010102434.ncn3mxk7cesec6s5@box.shutemov.name>
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On Tue, Oct 10, 2023 at 11:12:35AM +0200, Borislav Petkov wrote:
> On Tue, Oct 10, 2023 at 11:40:41AM +0300, Kirill A. Shutemov wrote:
> > __VIRTUAL_MASK_SHIFT used in many places. I don't think it is good idea to
> > give up on patching completely.
> 
> Have you even looked at boot_cpu_has()'s asm?

Obviously not :/

Okay, as alternative, the patch below also make the issue go away.

But I am not sure it is fundamentaly better. boot_cpu_has() generates call
to __asan_load8_noabort(). I think it only works because all KASAN code
has ASAN instrumentation disabled.

diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
index de75306b932e..bfe97013abb0 100644
--- a/arch/x86/include/asm/kasan.h
+++ b/arch/x86/include/asm/kasan.h
@@ -12,8 +12,15 @@
  * for kernel really starts from compiler's shadow offset +
  * 'kernel address space start' >> KASAN_SHADOW_SCALE_SHIFT
  */
+
+#ifdef USE_EARLY_PGTABLE_L5
+#define __KASAN_VIRT_SHIFT	(__pgtable_l5_enabled ? 56 : 47)
+#else
+#define __KASAN_VIRT_SHIFT	(boot_cpu_has(X86_FEATURE_LA57) ? 56 : 47)
+#endif
+
 #define KASAN_SHADOW_START      (KASAN_SHADOW_OFFSET + \
-					((-1UL << __VIRTUAL_MASK_SHIFT) >> \
+					((-1UL << __KASAN_VIRT_SHIFT) >> \
 						KASAN_SHADOW_SCALE_SHIFT))
 /*
  * 47 bits for kernel address -> (47 - KASAN_SHADOW_SCALE_SHIFT) bits for shadow
-- 
  Kiryl Shutsemau / Kirill A. Shutemov