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Subject: Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation
To: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>,
 Dmitry Vyukov <dvyukov@google.com>, Jonathan Corbet <corbet@lwn.net>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon
 <will.deacon@arm.com>, Marc Zyngier <marc.zyngier@arm.com>,
 Christopher Li <sparse@chrisli.org>, Christoph Lameter <cl@linux.com>,
 Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>,
 Joonsoo Kim <iamjoonsoo.kim@lge.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Masahiro Yamada <yamada.masahiro@socionext.com>,
 Michal Marek <michal.lkml@markovi.net>, Mark Rutland <mark.rutland@arm.com>,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>,
 Yury Norov <ynorov@caviumnetworks.com>,
 Nick Desaulniers <ndesaulniers@google.com>,
 Suzuki K Poulose <suzuki.poulose@arm.com>,
 Kristina Martsenko <kristina.martsenko@arm.com>,
 Punit Agrawal <punit.agrawal@arm.com>, Dave Martin <Dave.Martin@arm.com>,
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 Julien Thierry <julien.thierry@arm.com>, Steve Capper
 <steve.capper@arm.com>, Tyler Baicar <tbaicar@codeaurora.org>,
 "Eric W . Biederman" <ebiederm@xmission.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
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 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 David Woodhouse <dwmw@amazon.co.uk>,
 Sandipan Das <sandipan@linux.vnet.ibm.com>, Kees Cook
 <keescook@chromium.org>, Herbert Xu <herbert@gondor.apana.org.au>,
 Geert Uytterhoeven <geert@linux-m68k.org>,
 Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
 kasan-dev <kasan-dev@googlegroups.com>, linux-doc@vger.kernel.org,
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 Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>, Kees Cook
 <keescook@google.com>, Jann Horn <jannh@google.com>,
 Mark Brand <markbrand@google.com>
References: <cover.1521828273.git.andreyknvl@google.com>
 <ba4a74ba1bc48dd66a3831143c3119d13c291fe3.1521828274.git.andreyknvl@google.com>
 <805d1e85-2d3c-2327-6e6c-f14a56dc0b67@virtuozzo.com>
 <CAAeHK+yg5ODeDy7k9fako5mcCLLnBrO729Zp_-UtDuzh3hZgZA@mail.gmail.com>
 <0c4397da-e231-0044-986f-b8468314be76@virtuozzo.com>
 <CAAeHK+xmCLe85_QNDam_BVTp9wVzjxgvko2+0JapJCzmciGa5g@mail.gmail.com>
From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <0857f052-a27a-501e-8923-c6f31510e4fe@virtuozzo.com>
Date: Thu, 5 Apr 2018 16:02:20 +0300
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On 04/04/2018 08:00 PM, Andrey Konovalov wrote:
> On Wed, Apr 4, 2018 at 2:39 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>>>>
>>>> You can save tag somewhere in page struct and make page_address() return tagged address.
>>>>
>>>> I'm not sure it might be even possible to squeeze the tag into page->flags on some configurations,
>>>> see include/linux/page-flags-layout.h
>>>
>>> One page can contain multiple objects with different tags, so we would
>>> need to save the tag for each of them.
>>
>> What do you mean? Slab page? The per-page tag is needed only for !PageSlab pages.
>> For slab pages we have kmalloc/kmem_cache_alloc() which already return properly tagged address.
>>
>> But the page allocator returns a pointer to struct page. One has to call page_address(page)
>> to use that page. Returning 'ignore-me'-tagged address from page_address() makes the whole
>> class of bugs invisible to KHWASAN. This is a serious downside comparing to classic KASAN which can
>> detect missuses of page allocator API.
> 
> Yes, slab page. Here's an example:
> 
> 1. do_get_write_access() allocates frozen_buffer with jbd2_alloc,
> which calls kmem_cache_alloc, and then saves the result to
> jh->b_frozen_data.
> 
> 2. jbd2_journal_write_metadata_buffer() takes the value of
> jh_in->b_frozen_data and calls virt_to_page() (and offset_in_page())
> on it.
> 
> 3. jbd2_journal_write_metadata_buffer() then calls kmap_atomic(),
> which calls page_address(), on the resulting page address.
> 
> The tag gets erased. The page belongs to slab and can contain multiple
> objects with different tags.
> 

I see. Ideally that kind of problem should be fixed by reworking/redesigning such code,
however jbd2_journal_write_metadata_buffer() is far from the only place which
does that trick. Fixing all of them would be a huge task probably, so ignoring such
accesses seems to be the only choice we have. 

Nevertheless, this doesn't mean that we should ignore *all* accesses to !slab memory.