[PATCH v4 07/17] 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           | 55 ++++++++++++++++++++++++++++++++++++++
 mm/kasan/khwasan.c         | 47 ++++++++++++++++++++++++++++++++
 4 files changed, 127 insertions(+)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 7a31e8ccbad2..e7f37c0b7e14 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -250,6 +250,8 @@ 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 336385baf926..d7624b879d86 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_HW */
+
+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_HW */
 
 #endif /* LINUX_KASAN_H */
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 19b950eaccff..a7cc27d96608 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -8,6 +8,10 @@
 #define KASAN_SHADOW_SCALE_SIZE (1UL << KASAN_SHADOW_SCALE_SHIFT)
 #define KASAN_SHADOW_MASK       (KASAN_SHADOW_SCALE_SIZE - 1)
 
+#define KHWASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
+#define KHWASAN_TAG_INVALID	0xFE /* inaccessible memory tag */
+#define KHWASAN_TAG_MAX		0xFD /* maximum value for random tags */
+
 #define KASAN_FREE_PAGE         0xFF  /* page was freed */
 #define KASAN_PAGE_REDZONE      0xFE  /* redzone for kmalloc_large allocations */
 #define KASAN_KMALLOC_REDZONE   0xFC  /* redzone inside slub object */
@@ -126,6 +130,57 @@ static inline void quarantine_reduce(void) { }
 static inline void quarantine_remove_cache(struct kmem_cache *cache) { }
 #endif
 
+#ifdef CONFIG_KASAN_HW
+
+#define KHWASAN_TAG_SHIFT 56
+#define KHWASAN_TAG_MASK (0xFFUL << KHWASAN_TAG_SHIFT)
+
+u8 random_tag(void);
+
+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);
+}
+
+#else /* CONFIG_KASAN_HW */
+
+static inline u8 random_tag(void)
+{
+	return 0;
+}
+
+static inline void *set_tag(const void *addr, u8 tag)
+{
+	return (void *)addr;
+}
+
+static inline u8 get_tag(const void *addr)
+{
+	return 0;
+}
+
+static inline void *reset_tag(const void *addr)
+{
+	return (void *)addr;
+}
+
+#endif /* CONFIG_KASAN_HW */
+
 /*
  * Exported functions for interfaces called from assembly or from generated
  * code. Declarations here to avoid warning about missing declarations.
diff --git a/mm/kasan/khwasan.c b/mm/kasan/khwasan.c
index e2c3a7f7fd1f..d34679b8f8c7 100644
--- a/mm/kasan/khwasan.c
+++ b/mm/kasan/khwasan.c
@@ -38,6 +38,53 @@
 #include "kasan.h"
 #include "../slab.h"
 
+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();
+}
+
+/*
+ * 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.
+ */
+u8 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 check_memory_region(unsigned long addr, size_t size, bool write,
 				unsigned long ret_ip)
 {
-- 
2.18.0.rc2.346.g013aa6912e-goog