[RFC PATCH v2 08/15] khwasan: add tag related helper functions

[Andrey Konovalov <andreyknvl@google.com>]

This commit adds a few helper functions, that are meant to be used to
work with tags embedded in the top byte of kernel pointers: to set, to
get or to reset (set to 0xff) the top byte.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 arch/arm64/mm/kasan_init.c |  2 ++
 include/linux/kasan.h      | 23 +++++++++++++++++
 mm/kasan/kasan.h           | 29 ++++++++++++++++++++++
 mm/kasan/khwasan.c         | 51 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 105 insertions(+)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index d4bceba60010..7fd9aee88069 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -247,6 +247,8 @@ void __init kasan_init(void)
 	memset(kasan_zero_page, KASAN_SHADOW_INIT, PAGE_SIZE);
 	cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
 
+	khwasan_init();
+
 	/* At this point kasan is fully initialized. Enable error messages */
 	init_task.kasan_depth = 0;
 	pr_info("KernelAddressSanitizer initialized\n");
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 700734dff218..a2869464a8be 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -155,6 +155,29 @@ static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
 
 #define KASAN_SHADOW_INIT 0xFF
 
+void khwasan_init(void);
+
+void *khwasan_set_tag(const void *addr, u8 tag);
+u8 khwasan_get_tag(const void *addr);
+void *khwasan_reset_tag(const void *ptr);
+
+#else /* CONFIG_KASAN_TAGS */
+
+static inline void khwasan_init(void) { }
+
+static inline void *khwasan_set_tag(const void *addr, u8 tag)
+{
+	return (void *)addr;
+}
+static inline u8 khwasan_get_tag(const void *addr)
+{
+	return 0xFF;
+}
+static inline void *khwasan_reset_tag(const void *ptr)
+{
+	return (void *)ptr;
+}
+
 #endif /* CONFIG_KASAN_TAGS */
 
 #endif /* LINUX_KASAN_H */
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 2be31754278e..c715b44c4780 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -113,6 +113,35 @@ void kasan_report(unsigned long addr, size_t size,
 		bool is_write, unsigned long ip);
 void kasan_report_invalid_free(void *object, unsigned long ip);
 
+#define KHWASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
+#define KHWASAN_TAG_INVALID	0xFE /* redzone or free memory tag */
+#define KHWASAN_TAG_MAX		0xFD /* maximum value for random tags */
+
+#define KHWASAN_TAG_SHIFT 56
+#define KHWASAN_TAG_MASK (0xFFUL << KHWASAN_TAG_SHIFT)
+
+static inline void *set_tag(const void *addr, u8 tag)
+{
+	u64 a = (u64)addr;
+
+	a &= ~KHWASAN_TAG_MASK;
+	a |= ((u64)tag << KHWASAN_TAG_SHIFT);
+
+	return (void *)a;
+}
+
+static inline u8 get_tag(const void *addr)
+{
+	return (u8)((u64)addr >> KHWASAN_TAG_SHIFT);
+}
+
+static inline void *reset_tag(const void *addr)
+{
+	return set_tag(addr, KHWASAN_TAG_KERNEL);
+}
+
+void khwasan_report_invalid_free(void *object, unsigned long ip);
+
 #if defined(CONFIG_SLAB) || defined(CONFIG_SLUB)
 void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache);
 void quarantine_reduce(void);
diff --git a/mm/kasan/khwasan.c b/mm/kasan/khwasan.c
index 24d75245e9d0..da4b17997c71 100644
--- a/mm/kasan/khwasan.c
+++ b/mm/kasan/khwasan.c
@@ -39,6 +39,57 @@
 #include "kasan.h"
 #include "../slab.h"
 
+int khwasan_enabled;
+
+static DEFINE_PER_CPU(u32, prng_state);
+
+void khwasan_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		per_cpu(prng_state, cpu) = get_random_u32();
+	}
+	WRITE_ONCE(khwasan_enabled, 1);
+}
+
+/*
+ * If a preemption happens between this_cpu_read and this_cpu_write, the only
+ * side effect is that we'll give a few allocated in different contexts objects
+ * the same tag. Since KHWASAN is meant to be used a probabilistic bug-detection
+ * debug feature, this doesn’t have significant negative impact.
+ *
+ * Ideally the tags use strong randomness to prevent any attempts to predict
+ * them during explicit exploit attempts. But strong randomness is expensive,
+ * and we did an intentional trade-off to use a PRNG. This non-atomic RMW
+ * sequence has in fact positive effect, since interrupts that randomly skew
+ * PRNG at unpredictable points do only good.
+ */
+static inline u8 khwasan_random_tag(void)
+{
+	u32 state = this_cpu_read(prng_state);
+
+	state = 1664525 * state + 1013904223;
+	this_cpu_write(prng_state, state);
+
+	return (u8)state % (KHWASAN_TAG_MAX + 1);
+}
+
+void *khwasan_set_tag(const void *addr, u8 tag)
+{
+	return set_tag(addr, tag);
+}
+
+u8 khwasan_get_tag(const void *addr)
+{
+	return get_tag(addr);
+}
+
+void *khwasan_reset_tag(const void *addr)
+{
+	return reset_tag(addr);
+}
+
 void kasan_unpoison_shadow(const void *address, size_t size)
 {
 }
-- 
2.17.0.rc0.231.g781580f067-goog