Re: [PATCH v3 mm 08/13] kasan, mm: optimize krealloc poisoning

Marco Elver
2021-02-08T10:59:07Z

On Fri, Feb 05, 2021 at 06:34PM +0100, Andrey Konovalov wrote:
> Currently, krealloc() always calls ksize(), which unpoisons the whole
> object including the redzone. This is inefficient, as kasan_krealloc()
> repoisons the redzone for objects that fit into the same buffer.
>
> This patch changes krealloc() instrumentation to use uninstrumented
> __ksize() that doesn't unpoison the memory. Instead, kasan_kreallos()
> is changed to unpoison the memory excluding the redzone.
>
> For objects that don't fit into the old allocation, this patch disables
> KASAN accessibility checks when copying memory into a new object instead
> of unpoisoning it.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Marco Elver <elver@google.com>

Clarification below.

> ---
> mm/kasan/common.c | 12 ++++++++++--
> mm/slab_common.c | 20 ++++++++++++++------
> 2 files changed, 24 insertions(+), 8 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 7ea643f7e69c..a8a67dca5e55 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -476,7 +476,7 @@ static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
>
> /*
> * The object has already been unpoisoned by kasan_slab_alloc() for
> - * kmalloc() or by ksize() for krealloc().
> + * kmalloc() or by kasan_krealloc() for krealloc().
> */
>
> /*
> @@ -526,7 +526,7 @@ void * __must_check __kasan_kmalloc_large(const void *ptr, size_t size,
>
> /*
> * The object has already been unpoisoned by kasan_alloc_pages() for
> - * alloc_pages() or by ksize() for krealloc().
> + * alloc_pages() or by kasan_krealloc() for krealloc().
> */
>
> /*
> @@ -554,8 +554,16 @@ void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flag
> if (unlikely(object == ZERO_SIZE_PTR))
> return (void *)object;
>
> + /*
> + * Unpoison the object's data.
> + * Part of it might already have been unpoisoned, but it's unknown
> + * how big that part is.
> + */
> + kasan_unpoison(object, size);
> +
> page = virt_to_head_page(object);
>
> + /* Piggy-back on kmalloc() instrumentation to poison the redzone. */
> if (unlikely(!PageSlab(page)))
> return __kasan_kmalloc_large(object, size, flags);
> else
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index dad70239b54c..60a2f49df6ce 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1140,19 +1140,27 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
> void *ret;
> size_t ks;
>
> - if (likely(!ZERO_OR_NULL_PTR(p)) && !kasan_check_byte(p))
> - return NULL;
> -
> - ks = ksize(p);
> + /* Don't use instrumented ksize to allow precise KASAN poisoning. */
> + if (likely(!ZERO_OR_NULL_PTR(p))) {
> + if (!kasan_check_byte(p))
> + return NULL;

Just checking: Check byte returns true if the object is not tracked by KASAN, right? I.e. if it's a KFENCE object, kasan_check_byte() always returns true.

> + ks = kfence_ksize(p) ?: __ksize(p);
> + } else
> + ks = 0;
>
> + /* If the object still fits, repoison it precisely. */
> if (ks >= new_size) {
> p = kasan_krealloc((void *)p, new_size, flags);
> return (void *)p;
> }
>
> ret = kmalloc_track_caller(new_size, flags);
> - if (ret && p)
> - memcpy(ret, p, ks);
> + if (ret && p) {
> + /* Disable KASAN checks as the object's redzone is accessed. */
> + kasan_disable_current();
> + memcpy(ret, kasan_reset_tag(p), ks);
> + kasan_enable_current();
> + }
>
> return ret;
> }
> --
> 2.30.0.365.g02bc693789-goog
>