kasan: add tag related helper functions

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 the top byte.

Link: http://lkml.kernel.org/r/f6c6437bb8e143bc44f42c3c259c62e734be7935.1544099024.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index 8758bb008436d4e3fb6a5b119c55fa07d0186bbb..b52aacd2c5261ada9d417f2a6de2dbb96658ff1c 100644 (file)
--- a/arch/arm64/include/asm/kasan.h
+++ b/arch/arm64/include/asm/kasan.h
@@ -4,12 +4,16 @@
 
 #ifndef __ASSEMBLY__
 
-#ifdef CONFIG_KASAN
-
 #include <linux/linkage.h>
 #include <asm/memory.h>
 #include <asm/pgtable-types.h>
 
+#define arch_kasan_set_tag(addr, tag)  __tag_set(addr, tag)
+#define arch_kasan_reset_tag(addr)     __tag_reset(addr)
+#define arch_kasan_get_tag(addr)       __tag_get(addr)
+
+#ifdef CONFIG_KASAN
+
 /*
  * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
  * KASAN_SHADOW_END: KASAN_SHADOW_START + 1/N of kernel virtual addresses,

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index e73bb89d6141da4edad59b657f11e47311964c43..25b46f88726c177d92ec7ab568d8a7eddc8d931f 100644 (file)
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -226,6 +226,18 @@ extern u64                 vabits_user;
 #define untagged_addr(addr)    \
        ((__typeof__(addr))sign_extend64((u64)(addr), 55))
 
+#ifdef CONFIG_KASAN_SW_TAGS
+#define __tag_shifted(tag)     ((u64)(tag) << 56)
+#define __tag_set(addr, tag)   (__typeof__(addr))( \
+               ((u64)(addr) & ~__tag_shifted(0xff)) | __tag_shifted(tag))
+#define __tag_reset(addr)      untagged_addr(addr)
+#define __tag_get(addr)                (__u8)((u64)(addr) >> 56)
+#else
+#define __tag_set(addr, tag)   (addr)
+#define __tag_reset(addr)      (addr)
+#define __tag_get(addr)                0
+#endif
+
 /*
  * Physical vs virtual RAM address space conversion.  These are
  * private definitions which should NOT be used outside memory.h

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 38fa4bba92796f34a801f4253560e6bba2e90ae5..3e142add890b690e900ea85b400dd4cd7672b34a 100644 (file)
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -252,6 +252,8 @@ void __init kasan_init(void)
        memset(kasan_early_shadow_page, KASAN_SHADOW_INIT, PAGE_SIZE);
        cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
 
+       kasan_init_tags();
+
        /* 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 c56af24bd3e79920f64df4cb2ccf2da638cd7ac3..a477ce2abdc99c5f2b5cd20a362fad3bf8f194f4 100644 (file)
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -169,6 +169,19 @@ static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
 
 #define KASAN_SHADOW_INIT 0xFF
 
+void kasan_init_tags(void);
+
+void *kasan_reset_tag(const void *addr);
+
+#else /* CONFIG_KASAN_SW_TAGS */
+
+static inline void kasan_init_tags(void) { }
+
+static inline void *kasan_reset_tag(const void *addr)
+{
+       return (void *)addr;
+}
+
 #endif /* CONFIG_KASAN_SW_TAGS */
 
 #endif /* LINUX_KASAN_H */

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 19b950eaccffd40917c557b91e893949f32790e1..b080b8d92812b918ac956086d373f548b776e3b5 100644 (file)
--- 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 KASAN_TAG_KERNEL       0xFF /* native kernel pointers tag */
+#define KASAN_TAG_INVALID      0xFE /* inaccessible memory tag */
+#define KASAN_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 +130,33 @@ static inline void quarantine_reduce(void) { }
 static inline void quarantine_remove_cache(struct kmem_cache *cache) { }
 #endif
 
+#ifdef CONFIG_KASAN_SW_TAGS
+
+u8 random_tag(void);
+
+#else
+
+static inline u8 random_tag(void)
+{
+       return 0;
+}
+
+#endif
+
+#ifndef arch_kasan_set_tag
+#define arch_kasan_set_tag(addr, tag)  ((void *)(addr))
+#endif
+#ifndef arch_kasan_reset_tag
+#define arch_kasan_reset_tag(addr)     ((void *)(addr))
+#endif
+#ifndef arch_kasan_get_tag
+#define arch_kasan_get_tag(addr)       0
+#endif
+
+#define set_tag(addr, tag)     ((void *)arch_kasan_set_tag((addr), (tag)))
+#define reset_tag(addr)                ((void *)arch_kasan_reset_tag(addr))
+#define get_tag(addr)          arch_kasan_get_tag(addr)
+
 /*
  * Exported functions for interfaces called from assembly or from generated
  * code. Declarations here to avoid warning about missing declarations.

diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
index 04194923c543acfc5faf71e949430002db891cba..1c4e7ce2e6fe4bb09bf91c52629722adcb98dcf0 100644 (file)
--- a/mm/kasan/tags.c
+++ b/mm/kasan/tags.c
@@ -38,6 +38,43 @@
 #include "kasan.h"
 #include "../slab.h"
 
+static DEFINE_PER_CPU(u32, prng_state);
+
+void kasan_init_tags(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 tag-based KASAN 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 % (KASAN_TAG_MAX + 1));
+}
+
+void *kasan_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)
 {