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@xxxxxxxxxx>
---
arch/arm64/mm/kasan_init.c | 2 ++
include/linux/kasan.h | 23 +++++++++++++++++++
mm/kasan/khwasan.c | 47 ++++++++++++++++++++++++++++++++++++++
3 files changed, 72 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/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.rc1.244.gcf134e6275-goog
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