Re: [PATCH v2] kasan: test: add memcpy test that avoids out-of-bounds write

[Robin Murphy <robin.murphy@arm.com>]

On 2021-09-10 22:13, Peter Collingbourne wrote:
> With HW tag-based KASAN, error checks are performed implicitly by the
> load and store instructions in the memcpy implementation.  A failed check
> results in tag checks being disabled and execution will keep going. As a
> result, under HW tag-based KASAN, prior to commit 1b0668be62cf ("kasan:
> test: disable kmalloc_memmove_invalid_size for HW_TAGS"), this memcpy
> would end up corrupting memory until it hits an inaccessible page and
> causes a kernel panic.
> 
> This is a pre-existing issue that was revealed by commit 285133040e6c
> ("arm64: Import latest memcpy()/memmove() implementation") which changed
> the memcpy implementation from using signed comparisons (incorrectly,
> resulting in the memcpy being terminated early for negative sizes)
> to using unsigned comparisons.
> 
> It is unclear how this could be handled by memcpy itself in a reasonable
> way. One possibility would be to add an exception handler that would force
> memcpy to return if a tag check fault is detected -- this would make the
> behavior roughly similar to generic and SW tag-based KASAN. However,
> this wouldn't solve the problem for asynchronous mode and also makes
> memcpy behavior inconsistent with manually copying data.
> 
> This test was added as a part of a series that taught KASAN to detect
> negative sizes in memory operations, see commit 8cceeff48f23 ("kasan:
> detect negative size in memory operation function"). Therefore we
> should keep testing for negative sizes with generic and SW tag-based
> KASAN. But there is some value in testing small memcpy overflows, so
> let's add another test with memcpy that does not destabilize the kernel
> by performing out-of-bounds writes, and run it in all modes.

The only thing is, that's nonsense. You can't pass a negative size to 
memmove()/memcpy(), any more than you could pass a negative address. You 
can use the usual integer conversions to pass a very large size, but 
that's no different from just passing a very large size, and the 
language does not make any restrictions on the validity of very large 
sizes. Indeed in general a 32-bit program could legitimately memcpy() 
exactly half its address space to the other half, or memmove() a 3GB 
buffer a small distance.

I'm not sure what we're trying to enforce there, other than arbitrary 
restrictions on how we think it makes sense to call library functions. 
The only way to say that a size is actually invalid is if it leads to an 
out-of-bounds access relative to the source or destination buffer, but 
to provoke that the given size only ever needs to be at least 1 byte 
larger than the object - making it excessively large only generates 
excessively large numbers of invalid accesses, and I fail to see what 
use that has. By all means introduce KAROHWTIMSTCLFSAN, but I'm not 
convinced it's meaningfully within the scope of *address* sanitisation.

Thanks,
Robin.

> Link: https://linux-review.googlesource.com/id/I048d1e6a9aff766c4a53f989fb0c83de68923882
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> ---
>   lib/test_kasan.c | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 8835e0784578..aa8e42250219 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -493,7 +493,7 @@ static void kmalloc_oob_in_memset(struct kunit *test)
>   	kfree(ptr);
>   }
>   
> -static void kmalloc_memmove_invalid_size(struct kunit *test)
> +static void kmalloc_memmove_negative_size(struct kunit *test)
>   {
>   	char *ptr;
>   	size_t size = 64;
> @@ -515,6 +515,21 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
>   	kfree(ptr);
>   }
>   
> +static void kmalloc_memmove_invalid_size(struct kunit *test)
> +{
> +	char *ptr;
> +	size_t size = 64;
> +	volatile size_t invalid_size = size;
> +
> +	ptr = kmalloc(size, GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> +
> +	memset((char *)ptr, 0, 64);
> +	KUNIT_EXPECT_KASAN_FAIL(test,
> +		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
> +	kfree(ptr);
> +}
> +
>   static void kmalloc_uaf(struct kunit *test)
>   {
>   	char *ptr;
> @@ -1129,6 +1144,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
>   	KUNIT_CASE(kmalloc_oob_memset_4),
>   	KUNIT_CASE(kmalloc_oob_memset_8),
>   	KUNIT_CASE(kmalloc_oob_memset_16),
> +	KUNIT_CASE(kmalloc_memmove_negative_size),
>   	KUNIT_CASE(kmalloc_memmove_invalid_size),
>   	KUNIT_CASE(kmalloc_uaf),
>   	KUNIT_CASE(kmalloc_uaf_memset),
>