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Subject[PATCH v13 10/25] kasan: add tag related helper functions
Date
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.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
arch/arm64/include/asm/kasan.h | 8 +++++--
arch/arm64/include/asm/memory.h | 12 +++++++++++
arch/arm64/mm/kasan_init.c | 2 ++
include/linux/kasan.h | 13 ++++++++++++
mm/kasan/kasan.h | 31 +++++++++++++++++++++++++++
mm/kasan/tags.c | 37 +++++++++++++++++++++++++++++++++
6 files changed, 101 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index 8758bb008436..b52aacd2c526 100644
--- 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 e2c9857157f2..83c1366a1233 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -219,6 +219,18 @@ static inline unsigned long kaslr_offset(void)
#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 7a4a0904cac8..1df536bdabcb 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -253,6 +253,8 @@ 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 c56af24bd3e7..a477ce2abdc9 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..b080b8d92812 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 +130,33 @@ 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 04194923c543..1c4e7ce2e6fe 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)
{
--
2.20.0.rc1.387.gf8505762e3-goog
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