Re: [PATCH v2 1/1] kasan: Add memzero init for unaligned size under SLUB debug

[Yee Lee]

On Fri, 2021-06-25 at 17:03 +0300, Andrey Konovalov wrote:
> On Thu, Jun 24, 2021 at 2:26 PM <yee.lee@xxxxxxxxxxxx> wrote:
> > 
> > From: Yee Lee <yee.lee@xxxxxxxxxxxx>
> > 
> > Issue: when SLUB debug is on, hwtag kasan_unpoison() would
> > overwrite
> > the redzone of object with unaligned size.
> > 
> > An additional memzero_explicit() path is added to replacing init by
> > hwtag instruction for those unaligned size at SLUB debug mode.
> > 
> > Signed-off-by: Yee Lee <yee.lee@xxxxxxxxxxxx>
> > ---
> >  mm/kasan/kasan.h | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > index 8f450bc28045..d1054f35838f 100644
> > --- a/mm/kasan/kasan.h
> > +++ b/mm/kasan/kasan.h
> > @@ -387,6 +387,12 @@ static inline void kasan_unpoison(const void
> > *addr, size_t size, bool init)
> > 
> >         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
> >                 return;
> > +#if IS_ENABLED(CONFIG_SLUB_DEBUG)
> 
> Is this an issue only with SLUB? SLAB also uses redzones.
As I known, hw-tag kasan only works with SLUB.

> 
> > +       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> 
> This needs a comment along the lines of:
> 
> /* Explicitly initialize the memory with the precise object size to
> avoid overwriting the SLAB redzone. This disables initialization in
> the arch code and may thus lead to performance penalty. The penalty
> is
> accepted since SLAB redzones aren't enabled in production builds. */
Sure, will work on this.
> 
> > +               init = false;
> > +               memzero_explicit((void *)addr, size);
> > +       }
> > +#endif
> >         size = round_up(size, KASAN_GRANULE_SIZE);
> > 
> >         hw_set_mem_tag_range((void *)addr, size, tag, init);
> > --
> > 2.18.0
> >