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    SubjectRe: [PATCH v11 2/6] kasan: allow architectures to provide an outline readiness check
    On Fri, 19 Mar 2021 at 15:41, Daniel Axtens <dja@axtens.net> wrote:
    > Allow architectures to define a kasan_arch_is_ready() hook that bails
    > out of any function that's about to touch the shadow unless the arch
    > says that it is ready for the memory to be accessed. This is fairly
    > uninvasive and should have a negligible performance penalty.
    >
    > This will only work in outline mode, so an arch must specify
    > ARCH_DISABLE_KASAN_INLINE if it requires this.
    >
    > Cc: Balbir Singh <bsingharora@gmail.com>
    > Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    > Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    > Signed-off-by: Daniel Axtens <dja@axtens.net>
    >
    > --
    >
    > I discuss the justfication for this later in the series. Also,
    > both previous RFCs for ppc64 - by 2 different people - have
    > needed this trick! See:
    > - https://lore.kernel.org/patchwork/patch/592820/ # ppc64 hash series
    > - https://patchwork.ozlabs.org/patch/795211/ # ppc radix series
    > ---
    > include/linux/kasan.h | 4 ++++
    > mm/kasan/common.c | 4 ++++
    > mm/kasan/generic.c | 3 +++
    > mm/kasan/shadow.c | 4 ++++
    > 4 files changed, 15 insertions(+)
    >
    > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
    > index 8b3b99d659b7..6bd8343f0033 100644
    > --- a/include/linux/kasan.h
    > +++ b/include/linux/kasan.h

    Does kasan_arch_is_ready() need to be defined in the public interface
    of KASAN? Could it instead be moved to mm/kasan/kasan.h?

    > @@ -23,6 +23,10 @@ struct kunit_kasan_expectation {
    >
    > #endif
    >
    > +#ifndef kasan_arch_is_ready
    > +static inline bool kasan_arch_is_ready(void) { return true; }
    > +#endif
    > +
    > #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
    >
    > #include <linux/pgtable.h>
    > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
    > index 6bb87f2acd4e..f23a9e2dce9f 100644
    > --- a/mm/kasan/common.c
    > +++ b/mm/kasan/common.c
    > @@ -345,6 +345,10 @@ static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
    > if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
    > return false;
    >
    > + /* We can't read the shadow byte if the arch isn't ready */
    > + if (!kasan_arch_is_ready())
    > + return false;
    > +

    While it probably doesn't matter much, it seems this check could be
    moved up, rather than having it in the middle here.


    > if (!kasan_byte_accessible(tagged_object)) {
    > kasan_report_invalid_free(tagged_object, ip);
    > return true;
    > diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
    > index 53cbf28859b5..c3f5ba7a294a 100644
    > --- a/mm/kasan/generic.c
    > +++ b/mm/kasan/generic.c
    > @@ -163,6 +163,9 @@ static __always_inline bool check_region_inline(unsigned long addr,
    > size_t size, bool write,
    > unsigned long ret_ip)
    > {
    > + if (!kasan_arch_is_ready())
    > + return true;
    > +
    > if (unlikely(size == 0))
    > return true;
    >
    > diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
    > index 727ad4629173..1f650c521037 100644
    > --- a/mm/kasan/shadow.c
    > +++ b/mm/kasan/shadow.c
    > @@ -80,6 +80,10 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
    > */
    > addr = kasan_reset_tag(addr);
    >
    > + /* Don't touch the shadow memory if arch isn't ready */
    > + if (!kasan_arch_is_ready())
    > + return;
    > +
    > /* Skip KFENCE memory if called explicitly outside of sl*b. */
    > if (is_kfence_address(addr))
    > return;
    > --
    > 2.27.0
    >
    > --
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