Re: [PATCH] ARM: kasan: Only map modules if CONFIG_KASAN_VMALLOC=n

From: Linus Walleij <linus.walleij@linaro.org>
To: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: kasan-dev@googlegroups.com,
	Lecopzer Chen <lecopzer.chen@mediatek.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>,
	Russell King <linux@armlinux.org.uk>,
	 linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ARM: kasan: Only map modules if CONFIG_KASAN_VMALLOC=n
Date: Thu, 1 Sep 2022 15:12:12 +0200	[thread overview]
Message-ID: <CACRpkdarYrhtrv2W8+MQm6QNFkrqE-EUVz4cm7kGvsbWgBdk+Q@mail.gmail.com> (raw)
In-Reply-To: <ccde957b-20b1-2fd6-5c90-ad9ee4b8924c@nokia.com>

On Thu, Sep 1, 2022 at 10:42 AM Alexander Sverdlin
<alexander.sverdlin@nokia.com> wrote:

> >> -       create_mapping((void *)MODULES_VADDR, (void *)(PKMAP_BASE + PMD_SIZE));
> >> +       if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) && IS_ENABLED(CONFIG_MODULES))
> >> +               create_mapping((void *)MODULES_VADDR, (void *)(MODULES_END));
> > So the way I understand it is that modules are first and foremost loaded into
> > the area MODULES_VADDR .. MODULES_END, and then after that is out,
> > they get loaded into VMALLOC. See arch/arm/kernel/module.c, module_alloc().
>
> yes, but both areas are managed by __vmalloc_node_range().

Owww!

> > If you do this, how are the addresses between MODULES_VADDR..MODULES_END
> > shadowed when using CONFIG_KASAN_VMALLOC?
>
> That's the thing, __vmalloc_node_range() doesn't differentiate between address
> ranges and tries first to recreate [already existing] shadow mapping, and then
> vfree() unconditionally frees the mapping and the page.
>
> vmalloc() KASAN handling is generic, module_alloc() implemented via vmalloc()
> is however ARM-specific. Even though we could teach vmalloc() about MODULES_VADDR
> and MODULES_END (and don't call kasan_ instrumentation on these), but, this is
> ARM-specifics that it's used for this range.

OK I get it. Maybe this warrants a comment in the code explaining the
above behaviour (also in commitlog) so nobody gets confused.

With that:
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours.
Linus Walleij

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