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

[Will Deacon]

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