linux-kernel - Re: [PATCH v9 2/6] kasan: allow architectures to provide an outline readiness check

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Message-ID: <bc92b30f-f087-ced6-cf00-a62370eae733@csgroup.eu>
Date:   Tue, 1 Dec 2020 17:53:02 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Daniel Axtens <dja@...ens.net>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        kasan-dev@...glegroups.com, christophe.leroy@....fr,
        aneesh.kumar@...ux.ibm.com, bsingharora@...il.com
Cc:     "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v9 2/6] kasan: allow architectures to provide an outline
 readiness check



Le 01/12/2020 à 17:16, Daniel Axtens a écrit :
> 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
> HAVE_ARCH_NO_KASAN_INLINE if it requires this.
> 
> Cc: Balbir Singh <bsingharora@...il.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

Did I signed that off one day ? I can't remember.

Please update my email address, and maybe change it to a Suggested-by: ? I think the first 
Signed-off-by: has to be the author of the patch.

> Signed-off-by: Daniel Axtens <dja@...ens.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     | 10 ++++++++++
>   mm/kasan/generic.c    |  3 +++
>   3 files changed, 17 insertions(+)
> 
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 30d343b4a40a..3df66fdf6662 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -20,6 +20,10 @@ struct kunit_kasan_expectation {
>   	bool report_found;
>   };
>   
> +#ifndef kasan_arch_is_ready
> +static inline bool kasan_arch_is_ready(void)	{ return true; }
> +#endif
> +
>   extern unsigned char kasan_early_shadow_page[PAGE_SIZE];
>   extern pte_t kasan_early_shadow_pte[PTRS_PER_PTE];
>   extern pmd_t kasan_early_shadow_pmd[PTRS_PER_PMD];
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 950fd372a07e..ba7744d3e319 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -117,6 +117,9 @@ void kasan_poison_shadow(const void *address, size_t size, u8 value)
>   {
>   	void *shadow_start, *shadow_end;
>   
> +	if (!kasan_arch_is_ready())
> +		return;
> +
>   	/*
>   	 * Perform shadow offset calculation based on untagged address, as
>   	 * some of the callers (e.g. kasan_poison_object_data) pass tagged
> @@ -134,6 +137,9 @@ void kasan_unpoison_shadow(const void *address, size_t size)
>   {
>   	u8 tag = get_tag(address);
>   
> +	if (!kasan_arch_is_ready())
> +		return;
> +
>   	/*
>   	 * Perform shadow offset calculation based on untagged address, as
>   	 * some of the callers (e.g. kasan_unpoison_object_data) pass tagged
> @@ -406,6 +412,10 @@ static 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;
> +
>   	shadow_byte = READ_ONCE(*(s8 *)kasan_mem_to_shadow(object));
>   	if (shadow_invalid(tag, shadow_byte)) {
>   		kasan_report_invalid_free(tagged_object, ip);
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 248264b9cb76..e87404026b2b 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -169,6 +169,9 @@ static __always_inline bool check_memory_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;
>   
>