linux-kernel - Re: [PATCH v2] kasan: fix kasan_byte_accessible() to be consistent with actual checks

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Date:   Tue, 6 Apr 2021 01:46:09 +0200
From:   Marco Elver <elver@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Collingbourne <pcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kasan: fix kasan_byte_accessible() to be consistent
 with actual checks

On Tue, 6 Apr 2021 at 00:43, Andrey Konovalov <andreyknvl@...il.com> wrote:
> On Tue, Apr 6, 2021 at 12:07 AM Peter Collingbourne <pcc@...gle.com> wrote:
> >
> > We can sometimes end up with kasan_byte_accessible() being called
> > on non-slab memory. For example ksize() and krealloc() may end up
> > calling it on KFENCE allocated memory. In this case the memory will
> > be tagged with KASAN_SHADOW_INIT, which a subsequent patch ("kasan:
> > initialize shadow to TAG_INVALID for SW_TAGS") will set to the same
> > value as KASAN_TAG_INVALID, causing kasan_byte_accessible() to fail
> > when called on non-slab memory.
> >
> > This highlighted the fact that the check in kasan_byte_accessible()
> > was inconsistent with checks as implemented for loads and stores
> > (kasan_check_range() in SW tags mode and hardware-implemented
> > checks in HW tags mode). kasan_check_range() does not have a
> > check for KASAN_TAG_INVALID, and instead has a comparison against
> > KASAN_SHADOW_START. In HW tags mode, we do not have either, but we
> > do set TCR_EL1.TCMA which corresponds with the comparison against
> > KASAN_TAG_KERNEL.
> >
> > Therefore, update kasan_byte_accessible() for both SW and HW tags
> > modes to correspond with the respective checks on loads and stores.
> >
> > Link: https://linux-review.googlesource.com/id/Ic6d40803c57dcc6331bd97fbb9a60b0d38a65a36
> > Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> > ---
> >  mm/kasan/kasan.h   |  3 +--
> >  mm/kasan/sw_tags.c | 10 +++++++---
> >  2 files changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > index 8c55634d6edd..e18e8da35255 100644
> > --- a/mm/kasan/kasan.h
> > +++ b/mm/kasan/kasan.h
> > @@ -368,8 +368,7 @@ static inline bool kasan_byte_accessible(const void *addr)
> >         u8 ptr_tag = get_tag(addr);
> >         u8 mem_tag = hw_get_mem_tag((void *)addr);
> >
> > -       return (mem_tag != KASAN_TAG_INVALID) &&
> > -               (ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
> > +       return ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag;
> >  }
> >
> >  #else /* CONFIG_KASAN_HW_TAGS */
> > diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
> > index 94c2d33be333..00ae8913fc74 100644
> > --- a/mm/kasan/sw_tags.c
> > +++ b/mm/kasan/sw_tags.c
> > @@ -121,10 +121,14 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
> >  bool kasan_byte_accessible(const void *addr)
> >  {
> >         u8 tag = get_tag(addr);
> > -       u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
> > +       void *untagged_addr = kasan_reset_tag(addr);
> > +       u8 shadow_byte;
> >
> > -       return (shadow_byte != KASAN_TAG_INVALID) &&
> > -               (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
> > +       if (untagged_addr < kasan_shadow_to_mem((void *)KASAN_SHADOW_START))
> > +               return false;
> > +
> > +       shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
> > +       return tag == KASAN_TAG_KERNEL || tag == shadow_byte;
> >  }
> >
> >  #define DEFINE_HWASAN_LOAD_STORE(size)                                 \
> > --
> > 2.31.0.208.g409f899ff0-goog
> >
>
> Reviewed-by: Andrey Konovalov <andreyknvl@...il.com>
>
> Thank you, Peter!

Reviewed-by: Marco Elver <elver@...gle.com>

Thanks!