Re: [PATCH 07/12] kasan, mm: remove krealloc side-effect

[Marco Elver <elver@google.com>]

On Mon, Feb 01, 2021 at 08:43PM +0100, Andrey Konovalov wrote:
> Currently, if krealloc() is called on a freed object with KASAN enabled,
> it allocates and returns a new object, but doesn't copy any memory from
> the old one as ksize() returns 0. This makes a caller believe that
> krealloc() succeeded (KASAN report is printed though).
>
> This patch adds an accessibility check into __do_krealloc(). If the check
> fails, krealloc() returns NULL. This check duplicates the one in ksize();
> this is fixed in the following patch.

I think "side-effect" is ambiguous, because either way behaviour of
krealloc differs from a kernel with KASAN disabled. Something like
"kasan, mm: fail krealloc on already freed object" perhaps?

> This patch also adds a KASAN-KUnit test to check krealloc() behaviour
> when it's called on a freed object.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

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

> ---
>  lib/test_kasan.c | 20 ++++++++++++++++++++
>  mm/slab_common.c |  3 +++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 2bb52853f341..61bc894d9f7e 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -359,6 +359,25 @@ static void krealloc_pagealloc_less_oob(struct kunit *test)
>  					KMALLOC_MAX_CACHE_SIZE + 201);
>  }
>  
> +/*
> + * Check that krealloc() detects a use-after-free, returns NULL,
> + * and doesn't unpoison the freed object.
> + */
> +static void krealloc_uaf(struct kunit *test)
> +{
> +	char *ptr1, *ptr2;
> +	int size1 = 201;
> +	int size2 = 235;
> +
> +	ptr1 = kmalloc(size1, GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
> +	kfree(ptr1);
> +
> +	KUNIT_EXPECT_KASAN_FAIL(test, ptr2 = krealloc(ptr1, size2, GFP_KERNEL));
> +	KUNIT_ASSERT_PTR_EQ(test, (void *)ptr2, NULL);
> +	KUNIT_EXPECT_KASAN_FAIL(test, *(volatile char *)ptr1);
> +}
> +
>  static void kmalloc_oob_16(struct kunit *test)
>  {
>  	struct {
> @@ -1056,6 +1075,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
>  	KUNIT_CASE(krealloc_less_oob),
>  	KUNIT_CASE(krealloc_pagealloc_more_oob),
>  	KUNIT_CASE(krealloc_pagealloc_less_oob),
> +	KUNIT_CASE(krealloc_uaf),
>  	KUNIT_CASE(kmalloc_oob_16),
>  	KUNIT_CASE(kmalloc_uaf_16),
>  	KUNIT_CASE(kmalloc_oob_in_memset),
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 39d1a8ff9bb8..dad70239b54c 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1140,6 +1140,9 @@ 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);
>  
>  	if (ks >= new_size) {
> -- 
> 2.30.0.365.g02bc693789-goog
>