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

From: Andrey Konovalov <andreyknvl@google.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>,
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Subject: Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation
Date: Wed, 4 Apr 2018 19:00:35 +0200	[thread overview]
Message-ID: <CAAeHK+xmCLe85_QNDam_BVTp9wVzjxgvko2+0JapJCzmciGa5g@mail.gmail.com> (raw)
In-Reply-To: <0c4397da-e231-0044-986f-b8468314be76@virtuozzo.com>

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 don't see any possible way of khwasan_enabled being 0 here.
>>
>> Can't kmem_cache_alloc be called for the temporary caches that are
>> used before the slab allocator and kasan are initialized?
>
> kasan_init() runs before allocators are initialized.
> slab allocator obviously has to be initialized before it can be used.

Checked the code, it seems you are right. Boot caches are created
after kasan_init() is called. I will remove khwasan_enabled.

Thanks!