Re: [PATCH v13 19/25] kasan: add hooks implementation for tag-based mode

[Andrey Konovalov]

On Fri, Dec 14, 2018 at 1:34 PM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> On 12/12/18 3:04 PM, Andrey Konovalov wrote:
> > On Tue, Dec 11, 2018 at 5:22 PM Vincenzo Frascino
> > <vincenzo.frascino@xxxxxxx> wrote:
> >>
> >> Hi Andrey,
> >>
> >> On 06/12/2018 12:24, Andrey Konovalov wrote:
> >>> This commit adds tag-based KASAN specific hooks implementation and
> >>> adjusts common generic and tag-based KASAN ones.
> >>>
> >>> 1. When a new slab cache is created, tag-based KASAN rounds up the size of
> >>>    the objects in this cache to KASAN_SHADOW_SCALE_SIZE (== 16).
> >>>
> >>> 2. On each kmalloc tag-based KASAN generates a random tag, sets the shadow
> >>>    memory, that corresponds to this object to this tag, and embeds this
> >>>    tag value into the top byte of the returned pointer.
> >>>
> >>> 3. On each kfree tag-based KASAN poisons the shadow memory with a random
> >>>    tag to allow detection of use-after-free bugs.
> >>>
> >>> The rest of the logic of the hook implementation is very much similar to
> >>> the one provided by generic KASAN. Tag-based KASAN saves allocation and
> >>> free stack metadata to the slab object the same way generic KASAN does.
> >>>
> >>> Reviewed-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> >>> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> >>> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> >>> ---
> >>>  mm/kasan/common.c | 116 ++++++++++++++++++++++++++++++++++++++--------
> >>>  mm/kasan/kasan.h  |   8 ++++
> >>>  mm/kasan/tags.c   |  48 +++++++++++++++++++
> >>>  3 files changed, 153 insertions(+), 19 deletions(-)
> >>>
> >>
> >>
> >> [...]
> >>
> >>> @@ -265,6 +290,8 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
> >>>               return;
> >>>       }
> >>>
> >>> +     cache->align = round_up(cache->align, KASAN_SHADOW_SCALE_SIZE);
> >>> +
> >>
> >> Did you consider to set ARCH_SLAB_MINALIGN instead of this round up?
> >
> > I didn't know about this macro. Looks like we can use it to do the
> > same thing. Do you think it's a better solution to redefine
> > ARCH_SLAB_MINALIGN to KASAN_SHADOW_SCALE_SIZE for arm64 when tag-based
> > KASAN is enabled instead of adjusting cache->align in
> > kasan_cache_create?
> >
>
> Yes, I think it is better because in this way we do not need to add extra code
> to do the rounding.
>
> Curiosity, did you try your patches with SLUB red zoning enabled?
> Since the area used for the Redzone is just after the payload, aligning the
> object_size independently from the allocator could have side effects, at least
> if I understand well how the mechanism works.
>
> Setting ARCH_SLAB_MINALIGN should avoid this as well.
>
> What do you think?

Sounds good to me.

Andrew, how should proceed with this? Send another fixup patch or
resend the whole series?

>
> >>
> >> --
> >> Regards,
> >> Vincenzo
> >>
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> --
> Regards,
> Vincenzo
>
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