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

Andrey Konovalov
2020-10-01T22:00:27Z

On Thu, Oct 1, 2020 at 8:03 PM <elver@google.com> wrote:
>
> On Fri, Sep 25, 2020 at 12:50AM +0200, Andrey Konovalov wrote:
> > 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>
>
> Acked-by: Marco Elver <elver@google.com>
>
> I assume you have tested with the various SLUB debug options,

Yes.

> as well as
> things like memory initialization etc?

Will test before sending v4.

Thanks!

>
> > ---
> > 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
> >