[PATCH v11 2/6] kasan: allow architectures to provide an outline readiness check

[Daniel Axtens]

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 at gmail.com>
Cc: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
Suggested-by: Christophe Leroy <christophe.leroy at csgroup.eu>
Signed-off-by: Daniel Axtens <dja at 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
@@ -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;
+
 	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