[PATCH v3 37/39] kasan, slub: reset tags when accessing metadata

Andrey Konovalov
2020-09-24T22:52:41Z

SLUB allocator accesses metadata for slab objects, that may lie
out-of-bounds of the object itself, or be accessed when an object is freed.
Such accesses trigger tag faults and lead to false-positive reports with
hardware tag-based KASAN.

Software KASAN modes disable instrumentation for allocator code via
KASAN_SANITIZE Makefile macro, and rely on kasan_enable/disable_current()
annotations which are used to ignore KASAN reports.

With hardware tag-based KASAN neither of those options are available, as
it doesn't use compiler instrumetation, no tag faults are ignored, and MTE
is disabled after the first one.

Instead, reset tags when accessing metadata.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
Change-Id: I39f3c4d4f29299d4fbbda039bedf230db1c746fb
---
mm/page_poison.c | 2 +-
mm/slub.c | 25 ++++++++++++++-----------
2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/page_poison.c b/mm/page_poison.c
index 34b9181ee5d1..d90d342a391f 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -43,7 +43,7 @@ static void poison_page(struct page *page)

/* KASAN still think the page is in-use, so skip it. */
kasan_disable_current();
- memset(addr, PAGE_POISON, PAGE_SIZE);
+ memset(kasan_reset_tag(addr), PAGE_POISON, PAGE_SIZE);
kasan_enable_current();
kunmap_atomic(addr);
}
diff --git a/mm/slub.c b/mm/slub.c
index 68c02b2eecd9..f5b4bef3cd6c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -249,7 +249,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
{
#ifdef CONFIG_SLAB_FREELIST_HARDENED
/*
- * When CONFIG_KASAN_SW_TAGS is enabled, ptr_addr might be tagged.
+ * When CONFIG_KASAN_SW/HW_TAGS is enabled, ptr_addr might be tagged.
* Normally, this doesn't cause any issues, as both set_freepointer()
* and get_freepointer() are called with a pointer with the same tag.
* However, there are some issues with CONFIG_SLUB_DEBUG code. For
@@ -275,6 +275,7 @@ static inline void *freelist_dereference(const struct kmem_cache *s,

static inline void *get_freepointer(struct kmem_cache *s, void *object)
{
+ object = kasan_reset_tag(object);
return freelist_dereference(s, object + s->offset);
}

@@ -304,6 +305,7 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
BUG_ON(object == fp); /* naive detection of double free or corruption */
#endif

+ freeptr_addr = (unsigned long)kasan_reset_tag((void *)freeptr_addr);
*(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
}

@@ -538,8 +540,8 @@ static void print_section(char *level, char *text, u8 *addr,
unsigned int length)
{
metadata_access_enable();
- print_hex_dump(level, text, DUMP_PREFIX_ADDRESS, 16, 1, addr,
- length, 1);
+ print_hex_dump(level, kasan_reset_tag(text), DUMP_PREFIX_ADDRESS,
+ 16, 1, addr, length, 1);
metadata_access_disable();
}

@@ -570,7 +572,7 @@ static struct track *get_track(struct kmem_cache *s, void *object,

p = object + get_info_end(s);

- return p + alloc;
+ return kasan_reset_tag(p + alloc);
}

static void set_track(struct kmem_cache *s, void *object,
@@ -583,7 +585,8 @@ static void set_track(struct kmem_cache *s, void *object,
unsigned int nr_entries;

metadata_access_enable();
- nr_entries = stack_trace_save(p->addrs, TRACK_ADDRS_COUNT, 3);
+ nr_entries = stack_trace_save(kasan_reset_tag(p->addrs),
+ TRACK_ADDRS_COUNT, 3);
metadata_access_disable();

if (nr_entries < TRACK_ADDRS_COUNT)
@@ -747,7 +750,7 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct page *page,

static void init_object(struct kmem_cache *s, void *object, u8 val)
{
- u8 *p = object;
+ u8 *p = kasan_reset_tag(object);

if (s->flags & SLAB_RED_ZONE)
memset(p - s->red_left_pad, val, s->red_left_pad);
@@ -777,7 +780,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
u8 *addr = page_address(page);

metadata_access_enable();
- fault = memchr_inv(start, value, bytes);
+ fault = memchr_inv(kasan_reset_tag(start), value, bytes);
metadata_access_disable();
if (!fault)
return 1;
@@ -873,7 +876,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)

pad = end - remainder;
metadata_access_enable();
- fault = memchr_inv(pad, POISON_INUSE, remainder);
+ fault = memchr_inv(kasan_reset_tag(pad), POISON_INUSE, remainder);
metadata_access_disable();
if (!fault)
return 1;
@@ -1118,7 +1121,7 @@ void setup_page_debug(struct kmem_cache *s, struct page *page, void *addr)
return;

metadata_access_enable();
- memset(addr, POISON_INUSE, page_size(page));
+ memset(kasan_reset_tag(addr), POISON_INUSE, page_size(page));
metadata_access_disable();
}

@@ -2884,10 +2887,10 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
stat(s, ALLOC_FASTPATH);
}

- maybe_wipe_obj_freeptr(s, object);
+ maybe_wipe_obj_freeptr(s, kasan_reset_tag(object));

if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
- memset(object, 0, s->object_size);
+ memset(kasan_reset_tag(object), 0, s->object_size);

slab_post_alloc_hook(s, objcg, gfpflags, 1, &object);

--
2.28.0.681.g6f77f65b4e-goog