Re: [PATCH] kasan: [v2]unpoison use memzero to init unaligned object

[Marco Elver <elver@google.com>]

On Tue, 22 Jun 2021 at 12:48, Yee Lee <yee.lee@mediatek.com> wrote:
>
> On Tue, 2021-06-22 at 11:01 +0200, Marco Elver wrote:
> > On Tue, 22 Jun 2021 at 10:48, <yee.lee@mediatek.com> wrote:
> > >
> > > From: Yee Lee <yee.lee@mediatek.com>
> > >
> > > Follows the discussion:
> > > https://patchwork.kernel.org/project/linux-mediatek/list/?series=504439
> >
> > The info about the percentage of how frequent this is could have been
> > provided as a simple reply to the discussion.
> >
> > > This patch Add memzero_explict to initialize unaligned object.
> >
> > This patch does not apply to anything (I see it depends on the
> > previous patch).
> >
> > What you need to do is modify the original patch, and then send a
> > [PATCH v2] (git helps with that by passing --reroll-count or -v) that
> > applies cleanly to your base kernel tree.
> >
> > The commit message will usually end with '---' and then briefly
> > denote
> > what changed since the last version.
> >
> Got it.
>
> >
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#the-canonical-patch-format
> >
> > > Based on the integrateion of initialization in kasan_unpoison().
> > > The hwtag instructions, constrained with its granularity, has to
> > > overwrite the data btyes in unaligned objects. This would cause
> > > issue when it works with SLUB debug redzoning.
> > >
> > > In this patch, an additional initalizaing path is added for the
> > > unaligned objects. It contains memzero_explict() to clear out the
> > > data and disables its init flag for the following hwtag actions.
> > >
> > > In lab test, this path is executed about 1.1%(941/80854) within the
> > > overall kasan_unpoison during a non-debug booting process.
> >
> > Nice, thanks for the data. If it is somehow doable, however, I'd
> > still
> > recommend to additionally guard the new code path by a check if
> > debug-support was requested. Ideally with an IS_ENABLED() config
> > check
> > so that if it's a production kernel the branch is simply optimized
> > out
> > by the compiler.
>
> Does it mean the memzero code path would be applied only at
> CONFIG_DEBUG_SLUB enabled? It expects no other potential overwriting
> in non-debug kernel.

Yes, if the problem only occurs with slub debugging enabled.

> By the way, based on de-coupling principle, adding a specific
> conditional statement(is_enable slub_debug) in a primitive
> funciton(kasan_unpoison) is not neat. It may be more proper that the
> conditional statement be added in other procedures of slub alloc.

What do you have in mind?

Well, there is kmem_cache_debug_flags(). Perhaps there's a better
place to add the check?

> Thanks,
>
> BR,
> Yee
>
> >
> > > Lab test: QEMU5.2 (+mte) / linux kernel 5.13-rc7
> > >
> > > Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> > > ---
> > >  mm/kasan/kasan.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > > index d8faa64614b7..edc11bcc3ff3 100644
> > > --- a/mm/kasan/kasan.h
> > > +++ b/mm/kasan/kasan.h
> > > @@ -389,7 +389,7 @@ static inline void kasan_unpoison(const void
> > > *addr, size_t size, bool init)
> > >                 return;
> > >         if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> > >                 init = false;
> > > -               memset((void *)addr, 0, size);
> > > +               memzero_explicit((void *)addr, size);
> > >         }
> > >         size = round_up(size, KASAN_GRANULE_SIZE);
> > >         hw_set_mem_tag_range((void *)addr, size, tag, init);
> > > 2.18.0
> > >
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> > > .
>
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