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 <[email protected]>
---
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 9c64a00bbf9c..a51d6ea580b0 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..821f657d38b5 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;
+ ks = __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
On Mon, Feb 01, 2021 at 08:43PM +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 <[email protected]>
> ---
> 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 9c64a00bbf9c..a51d6ea580b0 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..821f657d38b5 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;
> + ks = __ksize(p);
> + } else
> + ks = 0;
>
This unfortunately broke KFENCE:
https://syzkaller.appspot.com/bug?extid=e444e1006d07feef0ef3 + various
other false positives.
We need to use ksize() here, as __ksize() is unaware of KFENCE. Or
somehow add the same check here that ksize() uses to get the real object
size.
> + /* 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
>