[PATCH 2/4] KASan: implement non-warning kasan_is_poisoned_shadow

[Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>]

We export kasan_poison_shadow and kasan_unpoison_shadow for use by
allocators, but provide no API to query whether a region is poisoned
without triggering a stack trace.

This can be useful however for more unconventional "allocators" like the
DMA API debug code, which can use it to detect double poisons/unpoisons
that should not occur if the DMA API is correctly used.

Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
---
 include/linux/kasan.h |  2 ++
 lib/kasan/generic.c   | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 5fa0bebb796b..7812e0fa805b 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -69,6 +69,7 @@ extern void kasan_disable_current(void);
 
 void kasan_poison_shadow(const void *address, size_t size, u8 value);
 void kasan_unpoison_shadow(const void *address, size_t size);
+int kasan_is_poisoned_shadow(const void *address, size_t size);
 
 bool kasan_save_enable_multi_shot(void);
 void kasan_restore_multi_shot(bool enabled);
@@ -77,6 +78,7 @@ void kasan_restore_multi_shot(bool enabled);
 
 static inline void kasan_poison_shadow(const void *address, size_t size, u8 value) {}
 static inline void kasan_unpoison_shadow(const void *address, size_t size) {}
+static inline int kasan_is_poisoned_shadow(const void *address, size_t size) { return -1; }
 
 static inline void kasan_enable_current(void) {}
 static inline void kasan_disable_current(void) {}
diff --git a/lib/kasan/generic.c b/lib/kasan/generic.c
index 3709b8da9aae..2432f9274401 100644
--- a/lib/kasan/generic.c
+++ b/lib/kasan/generic.c
@@ -177,6 +177,43 @@ static __always_inline bool check_memory_region_inline(unsigned long addr,
 	return !kasan_report(addr, size, write, ret_ip);
 }
 
+int kasan_is_poisoned_shadow(const void *_addr, size_t size)
+{
+	unsigned long addr = (unsigned long)_addr;
+	unsigned long ret;
+
+	if (!kasan_initialized)
+		return -1;
+
+	if (addr < kasan_shadow_start)
+		return -1;
+
+	if (addr > kasan_shadowed_end)
+		return -1;
+
+	if (unlikely(size == 0))
+		return -1;
+
+	if (unlikely(addr + size < addr))
+		return -1;
+
+	if (addr < kasan_shadow_base)
+		return -1;
+
+	ret = memory_is_nonzero(kasan_mem_to_shadow((void *)addr),
+			kasan_mem_to_shadow((void *)addr + size - 1) + 1);
+
+	if (unlikely(ret)) {
+		unsigned long last_byte = addr + size - 1;
+		s8 *last_shadow = (s8 *)kasan_mem_to_shadow((void *)last_byte);
+
+		if (unlikely(ret != (unsigned long)last_shadow ||
+			((long)(last_byte & KASAN_SHADOW_MASK) >= *last_shadow)))
+			return 1;
+	}
+	return 0;
+}
+
 void kasan_init(unsigned long membase, unsigned long memsize,
 		unsigned long shadow_base)
 {
-- 
2.39.2