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

On Mon, Feb 8, 2021 at 11:48 AM Marco Elver [off-list ref] wrote:

On Fri, Feb 05, 2021 at 06:34PM +0100, Andrey Konovalov wrote:

quoted

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 <redacted>

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

Clarification below.

quoted

---
 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.

kasan_check_byte() still performs the check, but since KFENCE objects
are never poisoned, the check always passes.

Thanks!

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