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

[Robin Murphy]

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@xxxxxxxxxx> 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.