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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Tue, 20 Jun 2023 15:56:10 +0200
Message-ID: <CA+fCnZdZ0=kKN6hE_OF7jV_r_FjTh3FZtkGHBD57ZfqCXStKHg@mail.gmail.com>
Subject: Re: [PATCH] kasan: add support for kasan.fault=panic_on_write
To:     Marco Elver <elver@google.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>
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On Tue, Jun 20, 2023 at 1:51=E2=80=AFPM Marco Elver <elver@google.com> wrot=
e:
>
> > 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.

Yeah, I get that it can be a bit better than async with a similar
slowdown, but there's little value in catching only reads from the
security standpoint.

> > 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 feature=
s:
>    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=3Dpanic_on_write`` results in panic for all
> +  asynchronously checked accesses.
>    Asymmetric mode: a bad access is detected synchronously on reads and
>    asynchronously on writes.

Could you move this to the section that describes the kasan.fault
flag? This seems more consistent.

Thanks!