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Date:   Tue, 10 Oct 2023 11:40:41 +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: <20231010084041.ut5sshyrofh27yyx@box.shutemov.name>
References: <20231010053716.2481-1-kirill.shutemov@linux.intel.com>
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On Tue, Oct 10, 2023 at 10:19:38AM +0200, Borislav Petkov wrote:
> On Tue, Oct 10, 2023 at 08:37:16AM +0300, Kirill A. Shutemov wrote:
> > On machines with 5-level paging, cpu_feature_enabled(X86_FEATURE_LA57)
> > got patched. It includes KASAN code, where KASAN_SHADOW_START depends on
> > __VIRTUAL_MASK_SHIFT, which is defined with the cpu_feature_enabled().
> 
> So use boot_cpu_has(X86_FEATURE_LA57).

__VIRTUAL_MASK_SHIFT used in many places. I don't think it is good idea to
give up on patching completely.

> > It seems that KASAN gets confused when apply_alternatives() patches the
> 
> It seems?

Admittedly, I don't understand KASAN well enough. I confirmed my idea
indirectly, by patching KASASN_SHADOW_START, as I mentioned.

> > KASAN_SHADOW_START users. A test patch that makes KASAN_SHADOW_START
> > static, by replacing __VIRTUAL_MASK_SHIFT with 56, fixes the issue.
> > 
> > During text_poke_early() in apply_alternatives(), KASAN should be
> > disabled. KASAN is already disabled in non-_early() text_poke().
> > 
> > It is unclear why the issue was not reported earlier. Bisecting does not
> > help. Older kernels trigger the issue less frequently, but it still
> > occurs. In the absence of any other clear offenders, the initial dynamic
> > 5-level paging support is to blame.
> 
> This whole thing sounds like it is still not really clear what is
> actually happening...

Maybe KASAN folks can help to understand the situation.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov