Re: [PATCH] lkdtm: fix potential use after free

[Mark Rutland]

Hi,

On Wed, Apr 24, 2019 at 05:59:52PM +0800, Weikang shi wrote:
> From: swkhack <swkhack@xxxxxxxxx>
> 
> The function lkdtm_READ_AFTER_FREE calls kfree(base) to free the memory
> of base. However, following kfree(base),
> it access the memory which base point to via base[offset]. This may result in a
> use-after-free bug. 

This is deliberate; the test is designed to provoke a use-after-free to
verify that this can be detected by tools such as KASAN.

> This patch moves kfree(base) after the dereference.

Doing this would break the test, so we should not make this change.

Was this something you spotted with a static analysis tool?

> Signed-off-by: swkhack <swkhack@xxxxxxxxx>

A signed-off-by line should have your real name rather than an alias.

Thanks,
Mark.

> ---
>  drivers/misc/lkdtm/heap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/lkdtm/heap.c b/drivers/misc/lkdtm/heap.c
> index 65026d7de..3e2f1580a 100644
> --- a/drivers/misc/lkdtm/heap.c
> +++ b/drivers/misc/lkdtm/heap.c
> @@ -77,7 +77,6 @@ void lkdtm_READ_AFTER_FREE(void)
>  	base[offset] = *val;
>  	pr_info("Value in memory before free: %x\n", base[offset]);
>  
> -	kfree(base);
>  
>  	pr_info("Attempting bad read from freed memory\n");
>  	saw = base[offset];
> @@ -88,6 +87,7 @@ void lkdtm_READ_AFTER_FREE(void)
>  	}
>  	pr_info("Memory was not poisoned\n");
>  
> +	kfree(base);
>  	kfree(val);
>  }
>  
> -- 
> 2.17.1
>