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Date:   Tue, 20 Jun 2023 13:51:05 +0200
From:   Marco Elver <elver@google.com>
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Taras Madan <tarasmadan@google.com>,
        Aleksandr Nogikh <nogikh@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Jonathan Corbet <corbet@lwn.net>, kasan-dev@googlegroups.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH] kasan: add support for kasan.fault=panic_on_write
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On Tue, Jun 20, 2023 at 01:45PM +0200, Andrey Konovalov wrote:
> On Tue, Jun 20, 2023 at 1:33 PM Marco Elver <elver@google.com> wrote:
> >
> > > On a related note, it looks like we have a typo in KASAN
> > > documentation: it states that asymm mode detects reads synchronously,
> > > and writes - asynchronously. Should be the reverse.
> >
> > This says the documentation is correct, and it's actually called for
> > writes: https://docs.kernel.org/arm64/memory-tagging-extension.html#tag-check-faults
> >
> > Who is right?
> 
> Ah, right. I did a quick google to check when I was writing the
> response and found this: https://lwn.net/Articles/882963/. But looks
> like that cover letter is wrong and the documentation is right. I
> wonder what the point of the asymmetric mode is then.

Maybe not as strong, but asymm mode makes sense from a microarch point
of view, where writes are always committed into a store buffer, but
reads can only commit when the data (incl. tag) is available.

> So the current code that you have should work perfectly. The only
> change I'd like to see is in the documentation.

Something like this (or more?)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 7f37a46af574..3c58392d931e 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -135,6 +135,8 @@ disabling KASAN altogether or controlling its features:
   fault occurs, the information is stored in hardware (in the TFSR_EL1
   register for arm64). The kernel periodically checks the hardware and
   only reports tag faults during these checks.
+  Note that ``kasan.fault=panic_on_write`` results in panic for all
+  asynchronously checked accesses.
   Asymmetric mode: a bad access is detected synchronously on reads and
   asynchronously on writes.