On Tue, Apr 10, 2018 at 6:31 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: > > > On 04/10/2018 07:07 PM, Andrey Konovalov wrote: >> On Fri, Apr 6, 2018 at 2:27 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: >>> On 04/06/2018 03:14 PM, Andrey Konovalov wrote: >>>> On Thu, Apr 5, 2018 at 3:02 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: >>>>> Nevertheless, this doesn't mean that we should ignore *all* accesses to !slab memory. >>>> >>>> So you mean we need to find a way to ignore accesses via pointers >>>> returned by page_address(), but still check accesses through all other >>>> pointers tagged with 0xFF? I don't see an obvious way to do this. I'm >>>> open to suggestions though. >>>> >>> >>> I'm saying that we need to ignore accesses to slab objects if pointer >>> to slab object obtained via page_address() + offset_in_page() trick, but don't ignore >>> anything else. >>> >>> So, save tag somewhere in page struct and poison shadow with that tag. Make page_address() to >>> return tagged address for all !PageSlab() pages. For PageSlab() pages page_address() should return >>> 0xff tagged address, so we could ignore such accesses. >> >> Which pages do you mean by !PageSlab()? > > Literally the "PageSlab(page) == false" pages. > >> The ones that are allocated and freed by pagealloc, but mot managed by the slab allocator? > > Yes. > >> Perhaps we should then add tagging to the pagealloc hook instead? >> > > Of course the tagging would be in kasan_alloc_pages(), where else that could be? And instead of what? I think I misunderstood your suggestion twice already :) To make it clear, you're suggesting: 1. Tag memory with a random tag in kasan_alloc_pages() and returned a tagged pointer from pagealloc. 2. Restore the tag for the pointers returned from page_address for !PageSlab() pages. 3. Set the tag to 0xff for the pointers returned from page_address for PageSlab() pages. Is this correct? In 2 instead of storing the tag in page_struct, we can just recover it from the shadow memory that corresponds to that page. What do you think about this?
