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Subject: Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on
 tag-based kasan
To:     Will Deacon <will@kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Walter Wu <walter-zh.wu@mediatek.com>,
        wsd_upstream@mediatek.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        linux-mediatek@lists.infradead.org,
        Alexander Potapenko <glider@google.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>
References: <20190819114420.2535-1-walter-zh.wu@mediatek.com>
 <20190819125625.bu3nbrldg7te5kwc@willie-the-truck>
 <20190819132347.GB9927@lakrids.cambridge.arm.com>
 <20190819133441.ejomv6cprdcz7hh6@willie-the-truck>
 <CAAeHK+w7cTGN8SgWQs0bPjPOrizqfUoMnJWTvUkCqv17Qt=3oQ@mail.gmail.com>
 <20190819142238.2jobs6vabkp2isg2@willie-the-truck>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <1ac7eb3e-156f-218c-8c5a-39a05dd46d55@arm.com>
Date:   Mon, 19 Aug 2019 15:35:16 +0100
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On 19/08/2019 15:22, Will Deacon wrote:
> On Mon, Aug 19, 2019 at 04:05:22PM +0200, Andrey Konovalov wrote:
>> On Mon, Aug 19, 2019 at 3:34 PM Will Deacon <will@kernel.org> wrote:
>>>
>>> On Mon, Aug 19, 2019 at 02:23:48PM +0100, Mark Rutland wrote:
>>>> On Mon, Aug 19, 2019 at 01:56:26PM +0100, Will Deacon wrote:
>>>>> On Mon, Aug 19, 2019 at 07:44:20PM +0800, Walter Wu wrote:
>>>>>> __arm_v7s_unmap() call iopte_deref() to translate pyh_to_virt address,
>>>>>> but it will modify pointer tag into 0xff, so there is a false positive.
>>>>>>
>>>>>> When enable tag-based kasan, phys_to_virt() function need to rewrite
>>>>>> its original pointer tag in order to avoid kasan report an incorrect
>>>>>> memory corruption.
>>>>>
>>>>> Hmm. Which tree did you see this on? We've recently queued a load of fixes
>>>>> in this area, but I /thought/ they were only needed after the support for
>>>>> 52-bit virtual addressing in the kernel.
>>>>
>>>> I'm seeing similar issues in the virtio blk code (splat below), atop of
>>>> the arm64 for-next/core branch. I think this is a latent issue, and
>>>> people are only just starting to test with KASAN_SW_TAGS.
>>>>
>>>> It looks like the virtio blk code will round-trip a SLUB-allocated pointer from
>>>> virt->page->virt, losing the per-object tag in the process.
>>>>
>>>> Our page_to_virt() seems to get a per-page tag, but this only makes
>>>> sense if you're dealing with the page allocator, rather than something
>>>> like SLUB which carves a page into smaller objects giving each object a
>>>> distinct tag.
>>>>
>>>> Any round-trip of a pointer from SLUB is going to lose the per-object
>>>> tag.
>>>
>>> Urgh, I wonder how this is supposed to work?
>>>
>>> If we end up having to check the KASAN shadow for *_to_virt(), then why
>>> do we need to store anything in the page flags at all? Andrey?
>>
>> As per 2813b9c0 ("kasan, mm, arm64: tag non slab memory allocated via
>> pagealloc") we should only save a non-0xff tag in page flags for non
>> slab pages.
> 
> Thanks, that makes sense. Hopefully the patch from Andrey R will solve
> both of the reported splats, since I'd not realised they were both on the
> kfree() path.
> 
>> Could you share your .config so I can reproduce this?
> 
> This is in the iopgtable code, so it's probably pretty tricky to trigger
> at runtime unless you have the write IOMMU hardware, unfortunately.

If simply freeing any entry from the l2_tables cache is sufficient, then 
the short-descriptor selftest should do the job, and that ought to run 
on anything (modulo insane RAM layouts).

Robin.