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

[Andrey Ryabinin]

On 8/19/19 4:34 PM, Will Deacon 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?
> 

We supposed to ignore pointers with 0xff tags. We do ignore them when memory access checked,
but not in kfree() path.
This untested patch should fix the issue:



---
 mm/kasan/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 895dc5e2b3d5..0a81cc328049 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -407,7 +407,7 @@ static inline bool shadow_invalid(u8 tag, s8 shadow_byte)
 		return shadow_byte < 0 ||
 			shadow_byte >= KASAN_SHADOW_SCALE_SIZE;
 	else
-		return tag != (u8)shadow_byte;
+		return (tag != KASAN_TAG_KERNEL) && (tag != (u8)shadow_byte);
 }
 
 static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
-- 
2.21.0