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authorAndrey Konovalov <andreyknvl@google.com>2018-12-28 00:30:16 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2018-12-28 12:11:43 -0800
commit3c9e3aa11094e821aff4a8f6812a6e032293dbc0 (patch)
tree6d4b0baeb13a2f51dc3b5dd2019b0133b0456ca2 /mm/kasan
parent9c23f84723d2bb5611a973f56f0952fa74f048f3 (diff)
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>
Diffstat (limited to 'mm/kasan')
-rw-r--r--mm/kasan/kasan.h31
-rw-r--r--mm/kasan/tags.c37
2 files changed, 68 insertions, 0 deletions
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)
{