diff mbox series

[v10,09/22] kasan: add tag related helper functions

Message ID b8c56d36b79eecf0c331a0a7a2df12632aefccc9.1541525354.git.andreyknvl@google.com
State New
Headers show
Series kasan: add software tag-based mode for arm64 | expand

Commit Message

Andrey Konovalov Nov. 6, 2018, 5:30 p.m. UTC 
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.

Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 arch/arm64/mm/kasan_init.c |  2 ++
 include/linux/kasan.h      | 13 +++++++++
 mm/kasan/kasan.h           | 55 ++++++++++++++++++++++++++++++++++++++
 mm/kasan/tags.c            | 37 +++++++++++++++++++++++++
 4 files changed, 107 insertions(+)

Comments

Mark Rutland Nov. 7, 2018, 5:23 p.m. UTC | #1 
On Tue, Nov 06, 2018 at 06:30:24PM +0100, Andrey Konovalov wrote:
> 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.
> 
> Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  arch/arm64/mm/kasan_init.c |  2 ++
>  include/linux/kasan.h      | 13 +++++++++
>  mm/kasan/kasan.h           | 55 ++++++++++++++++++++++++++++++++++++++
>  mm/kasan/tags.c            | 37 +++++++++++++++++++++++++
>  4 files changed, 107 insertions(+)
> 
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 18ebc8994a7b..370b19d0e2fb 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -249,6 +249,8 @@ void __init kasan_init(void)
>  	memset(kasan_zero_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 7f6574c35c62..4c9d6f9029f2 100644
> --- 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;
> +}
> +

> +#ifdef CONFIG_KASAN_SW_TAGS
> +
> +#define KASAN_PTR_TAG_SHIFT 56
> +#define KASAN_PTR_TAG_MASK (0xFFUL << KASAN_PTR_TAG_SHIFT)
> +
> +u8 random_tag(void);
> +
> +static inline void *set_tag(const void *addr, u8 tag)
> +{
> +	u64 a = (u64)addr;
> +
> +	a &= ~KASAN_PTR_TAG_MASK;
> +	a |= ((u64)tag << KASAN_PTR_TAG_SHIFT);
> +
> +	return (void *)a;
> +}
> +
> +static inline u8 get_tag(const void *addr)
> +{
> +	return (u8)((u64)addr >> KASAN_PTR_TAG_SHIFT);
> +}
> +
> +static inline void *reset_tag(const void *addr)
> +{
> +	return set_tag(addr, KASAN_TAG_KERNEL);
> +}

We seem to be duplicating this functionality in several places.

Could we please make it so that the arch code defines macros:

arch_kasan_set_tag(addr, tag)
arch_kasan_get_tag(addr)
arch_kasan_reset_tag(addr)

... and use thoses consistently rather than open-coding them?

> +
> +#else /* CONFIG_KASAN_SW_TAGS */
> +
> +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;
> +}

... these can be defined in linux/kasan.h as:

#define arch_kasan_set_tag(addr, tag)	(addr)
#define arch_kasan_get_tag(addr)	0
#define arch_kasan_reset_tag(addr)	(addr)

Thanks,
Mark.
Andrey Konovalov Nov. 14, 2018, 7:19 p.m. UTC | #2 
On Wed, Nov 7, 2018 at 6:23 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Tue, Nov 06, 2018 at 06:30:24PM +0100, Andrey Konovalov wrote:
>> 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.
>>
>> Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
>> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>> ---
>>  arch/arm64/mm/kasan_init.c |  2 ++
>>  include/linux/kasan.h      | 13 +++++++++
>>  mm/kasan/kasan.h           | 55 ++++++++++++++++++++++++++++++++++++++
>>  mm/kasan/tags.c            | 37 +++++++++++++++++++++++++
>>  4 files changed, 107 insertions(+)
>>
>> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
>> index 18ebc8994a7b..370b19d0e2fb 100644
>> --- a/arch/arm64/mm/kasan_init.c
>> +++ b/arch/arm64/mm/kasan_init.c
>> @@ -249,6 +249,8 @@ void __init kasan_init(void)
>>       memset(kasan_zero_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 7f6574c35c62..4c9d6f9029f2 100644
>> --- 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;
>> +}
>> +
>
>> +#ifdef CONFIG_KASAN_SW_TAGS
>> +
>> +#define KASAN_PTR_TAG_SHIFT 56
>> +#define KASAN_PTR_TAG_MASK (0xFFUL << KASAN_PTR_TAG_SHIFT)
>> +
>> +u8 random_tag(void);
>> +
>> +static inline void *set_tag(const void *addr, u8 tag)
>> +{
>> +     u64 a = (u64)addr;
>> +
>> +     a &= ~KASAN_PTR_TAG_MASK;
>> +     a |= ((u64)tag << KASAN_PTR_TAG_SHIFT);
>> +
>> +     return (void *)a;
>> +}
>> +
>> +static inline u8 get_tag(const void *addr)
>> +{
>> +     return (u8)((u64)addr >> KASAN_PTR_TAG_SHIFT);
>> +}
>> +
>> +static inline void *reset_tag(const void *addr)
>> +{
>> +     return set_tag(addr, KASAN_TAG_KERNEL);
>> +}
>
> We seem to be duplicating this functionality in several places.
>
> Could we please make it so that the arch code defines macros:
>
> arch_kasan_set_tag(addr, tag)
> arch_kasan_get_tag(addr)
> arch_kasan_reset_tag(addr)
>
> ... and use thoses consistently rather than open-coding them?
>
>> +
>> +#else /* CONFIG_KASAN_SW_TAGS */
>> +
>> +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;
>> +}
>
> ... these can be defined in linux/kasan.h as:
>
> #define arch_kasan_set_tag(addr, tag)   (addr)
> #define arch_kasan_get_tag(addr)        0
> #define arch_kasan_reset_tag(addr)      (addr)

Will do in v11, thanks!
diff mbox series

Patch

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 18ebc8994a7b..370b19d0e2fb 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -249,6 +249,8 @@  void __init kasan_init(void)
 	memset(kasan_zero_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 7f6574c35c62..4c9d6f9029f2 100644
--- 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 19b950eaccff..f16bee55b610 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 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 +130,57 @@  static inline void quarantine_reduce(void) { }
 static inline void quarantine_remove_cache(struct kmem_cache *cache) { }
 #endif
 
+#ifdef CONFIG_KASAN_SW_TAGS
+
+#define KASAN_PTR_TAG_SHIFT 56
+#define KASAN_PTR_TAG_MASK (0xFFUL << KASAN_PTR_TAG_SHIFT)
+
+u8 random_tag(void);
+
+static inline void *set_tag(const void *addr, u8 tag)
+{
+	u64 a = (u64)addr;
+
+	a &= ~KASAN_PTR_TAG_MASK;
+	a |= ((u64)tag << KASAN_PTR_TAG_SHIFT);
+
+	return (void *)a;
+}
+
+static inline u8 get_tag(const void *addr)
+{
+	return (u8)((u64)addr >> KASAN_PTR_TAG_SHIFT);
+}
+
+static inline void *reset_tag(const void *addr)
+{
+	return set_tag(addr, KASAN_TAG_KERNEL);
+}
+
+#else /* CONFIG_KASAN_SW_TAGS */
+
+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_SW_TAGS */
+
 /*
  * 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 04194923c543..700323946867 100644
--- 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)
 {