Re: [PATCH] kasan: add support for kasan.fault=panic_on_write

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
Date: Tue, 20 Jun 2023 13:32:57 +0200	[thread overview]
Message-ID: <CANpmjNOSnVNy14xAVe6UHD0eHuMpxweg86+mYLQHpLM1k0H_cg@mail.gmail.com> (raw)
In-Reply-To: <CA+fCnZdy4TmMacvsPkoenCynUYsyKZ+kU1fx7cDpbh_6=cEPAQ@mail.gmail.com>

On Tue, 20 Jun 2023 at 12:57, Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Wed, Jun 14, 2023 at 11:52 AM Marco Elver <elver@google.com> wrote:
> >
> > @@ -597,7 +614,11 @@ void kasan_report_async(void)
> >         pr_err("Asynchronous fault: no details available\n");
> >         pr_err("\n");
> >         dump_stack_lvl(KERN_ERR);
> > -       end_report(&flags, NULL);
> > +       /*
> > +        * Conservatively set is_write=true, because no details are available.
> > +        * In this mode, kasan.fault=panic_on_write is like kasan.fault=panic.
> > +        */
> > +       end_report(&flags, NULL, true);
>
> Hi Marco,
>
> When asymm mode is enabled, kasan_report_async should only be called
> for read accesses. I think we could check the mode and panic
> accordingly.

How do we check the mode, and how do we prove it's only called for
read accesses?

> Please also update the documentation to describe the flag behavior wrt
> async/asymm modes.

Will do.

> 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?