Re: [PATCH] lkdtm: fix ACCESS_USERSPACE test

[Kees Cook]

On Wed, Oct 28, 2015 at 5:47 AM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> Add a copy_to_user() call to the ACCESS_USERSPACE test
> prior to attempting direct dereferencing of the user
> address to ensure the page is present.  Otherwise,
> a fault occurs on arm kernels even prior to the introduction
> of CONFIG_CPU_SW_DOMAIN_PAN, and there is no difference in
> behavior for CONFIG_CPU_SW_DOMAIN_PAN=n vs CONFIG_CPU_SW_DOMAIN_PAN=y.
>
> Before this change, for any value of CONFIG_CPU_SW_DOMAIN_PAN:
> lkdtm: Performing direct entry ACCESS_USERSPACE
> lkdtm: attempting bad read at b6fe8000
> Unable to handle kernel paging request at virtual address b6fe8000
>
> After this change, for CONFIG_CPU_SW_DOMAIN_PAN=n:
> lkdtm: Performing direct entry ACCESS_USERSPACE
> lkdtm: attempting bad read at b6efc000
> lkdtm: attempting bad write at b6efc000
>
> After this change, for CONFIG_CPU_SW_DOMAIN_PAN=y:
> lkdtm: Performing direct entry ACCESS_USERSPACE
> lkdtm: attempting bad read at b6f7d000
> Unhandled fault: page domain fault (0x01b) at 0xb6f7d000
> ...
>
> Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>

Great catch! In looking at this test again, I wonder if
ACCESS_USERSPACE should be split so that read and write can be tested
separately...

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  drivers/misc/lkdtm.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
> index b5abe34..11fdadc 100644
> --- a/drivers/misc/lkdtm.c
> +++ b/drivers/misc/lkdtm.c
> @@ -472,7 +472,7 @@ static void lkdtm_do_action(enum ctype which)
>                 break;
>         }
>         case CT_ACCESS_USERSPACE: {
> -               unsigned long user_addr, tmp;
> +               unsigned long user_addr, tmp = 0;
>                 unsigned long *ptr;
>
>                 user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
> @@ -483,6 +483,12 @@ static void lkdtm_do_action(enum ctype which)
>                         return;
>                 }
>
> +               if (copy_to_user((void __user *)user_addr, &tmp, sizeof(tmp))) {
> +                       pr_warn("copy_to_user failed\n");
> +                       vm_munmap(user_addr, PAGE_SIZE);
> +                       return;
> +               }
> +
>                 ptr = (unsigned long *)user_addr;
>
>                 pr_info("attempting bad read at %p\n", ptr);
> --
> 2.4.3
>



-- 
Kees Cook
Chrome OS Security
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