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

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Date:   Mon,  5 Apr 2021 14:43:21 -0700
From:   Peter Collingbourne <pcc@...gle.com>
To:     Marco Elver <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>
Cc:     Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] kasan: fix kasan_byte_accessible() to be consistent with
 actual checks

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 | 8 +++++---
 2 files changed, 6 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..914225eeda99 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -121,10 +121,12 @@ 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 = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));

-	return (shadow_byte != KASAN_TAG_INVALID) &&
-		(tag == KASAN_TAG_KERNEL || tag == shadow_byte);
+	return untagged_addr >=
+		       kasan_shadow_to_mem((void *)KASAN_SHADOW_START) &&
+	       (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
 }

 #define DEFINE_HWASAN_LOAD_STORE(size)					\
-- 
2.31.0.208.g409f899ff0-goog