Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation

From: Andrey Konovalov <andreyknvl@google.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
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Subject: Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation
Date: Fri, 6 Apr 2018 14:14:03 +0200	[thread overview]
Message-ID: <CAAeHK+xnHeznZwofNQVDcBCCMnaEQ6fcRxOcrFM-qQFUsZ51Rg@mail.gmail.com> (raw)
In-Reply-To: <0857f052-a27a-501e-8923-c6f31510e4fe@virtuozzo.com>

On Thu, Apr 5, 2018 at 3:02 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> 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.

So you mean we need to find a way to ignore accesses via pointers
returned by page_address(), but still check accesses through all other
pointers tagged with 0xFF? I don't see an obvious way to do this. I'm
open to suggestions though.