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

[Robin Murphy <robin.murphy@arm.com>]

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.