[PATCH v10 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 bb862d1f0e15..d314c0fa5804 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 a390fae9d64b..871ceefd723d 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -348,6 +348,10 @@ 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;
+
 	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 2e55e0f82f39..718c171584e3 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 de6b3f074742..0aafc2d5138f 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -85,6 +85,10 @@ void kasan_poison(const void *address, size_t size, u8 value)
 	address = kasan_reset_tag(address);
 	size = round_up(size, KASAN_GRANULE_SIZE);
 
+	/* 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(address))
 		return;
-- 
2.27.0