linux-kernel - Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on tag-based kasan

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Date:   Mon, 19 Aug 2019 15:22:38 +0100
From:   Will Deacon <will@...nel.org>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Walter Wu <walter-zh.wu@...iatek.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        wsd_upstream@...iatek.com, LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-mediatek@...ts.infradead.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on
 tag-based kasan

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@...nel.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.

Will