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Date:   Mon, 19 Aug 2019 14:34:42 +0100
From:   Will Deacon <will@kernel.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Walter Wu <walter-zh.wu@mediatek.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        wsd_upstream@mediatek.com, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com, linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on
 tag-based kasan
Message-ID: <20190819133441.ejomv6cprdcz7hh6@willie-the-truck>
References: <20190819114420.2535-1-walter-zh.wu@mediatek.com>
 <20190819125625.bu3nbrldg7te5kwc@willie-the-truck>
 <20190819132347.GB9927@lakrids.cambridge.arm.com>
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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?

Will