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 <redacted>
---
 arch/arm64/mm/kasan_init.c |  2 ++
 include/linux/kasan.h      | 29 +++++++++++++++++
 mm/kasan/kasan.h           | 55 ++++++++++++++++++++++++++++++++
 mm/kasan/khwasan.c         | 65 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 151 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 1c31bb089154..1f852244e739 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h

@@ -166,6 +166,35 @@ static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}

 #define KASAN_SHADOW_INIT 0xFF

+void khwasan_init(void);
+
+void *khwasan_reset_tag(const void *addr);
+
+void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr);
+void *khwasan_preset_slab_tag(struct kmem_cache *cache, unsigned int idx,
+                                       const void *addr);
+
+#else /* CONFIG_KASAN_HW */
+
+static inline void khwasan_init(void) { }
+
+static inline void *khwasan_reset_tag(const void *addr)
+{
+       return (void *)addr;
+}
+
+static inline void *khwasan_preset_slub_tag(struct kmem_cache *cache,
+                                               const void *addr)
+{
+       return (void *)addr;
+}
+
+static inline void *khwasan_preset_slab_tag(struct kmem_cache *cache,
+                                       unsigned int idx, const void *addr)
+{
+       return (void *)addr;
+}
+
 #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..9d91bf3c8246 100644
--- a/mm/kasan/khwasan.c
+++ b/mm/kasan/khwasan.c

@@ -38,6 +38,71 @@
 #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_reset_tag(const void *addr)
+{
+       return reset_tag(addr);
+}
+
+void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr)