Re: [PATCH] slub: untag object before slab end

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Feb 13, 2019 at 10:12 PM Qian Cai <cai@lca.pw> wrote:
>
> On Wed, 2019-02-13 at 11:31 +0100, Andrey Konovalov wrote:
> > On Wed, Feb 13, 2019 at 3:06 AM Qian Cai <cai@lca.pw> wrote:
> > >
> > > get_freepointer() could return NULL if there is no more free objects in
> > > the slab. However, it could return a tagged pointer (like
> > > 0x2200000000000000) with KASAN_SW_TAGS which would escape the NULL
> > > object checking in check_valid_pointer() and trigger errors below, so
> > > untag the object before checking for a NULL object there.
> >
> > I think this solution is just masking the issue. get_freepointer()
> > shouldn't return tagged NULLs. Apparently when we save a freelist
> > pointer, the object where the pointer gets written is tagged
> > differently, than this same object when the pointer gets read. I found
> > one case where this happens (the last patch out my 5 patch series),
> > but apparently there are more.
>
> Well, the problem is that,
>
> __free_slab
>   for_each_object(p, s, page_address(page) [1]
>     check_object(s, page, p ...)
>       get_freepointer(s, p)
>
> [1]: p += s->size
>
> page_address() tags the address using page_kasan_tag(page), so each "p" here has
> that tag.

Ah, I see what the issue is. With those 5 patches page_address()
should return 0xff-tagged pointer here, but when we set_freepointer()
the tag indeed might be different. OK, I think that patch that you
linked below is the better way to deal with this. I've added a
detailed comment to it and sent it.

Thanks!

>
> However, at beginning in allocate_slab(), it tags each object with a random tag,
> and then calls set_freepointer(s, p, NULL)
>
> As the result, get_freepointer() returns a tagged NULL because it never be able
> to obtain the original tag of the object anymore, and this calculation is now
> wrong.
>
> return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
>
> This also explain why this patch also works, as it unifies the tags.
>
> https://marc.info/?l=linux-mm&m=154955366113951&w=2
>
>