Re: [PATCHv2] lkdtm: Add READ_AFTER_FREE test

[Kees Cook]

On Thu, Feb 18, 2016 at 5:15 PM, Laura Abbott <labbott@xxxxxxxxxxxxxxxxx> wrote:
>
> In a similar manner to WRITE_AFTER_FREE, add a READ_AFTER_FREE
> test to test free poisoning features. Sample output when
> no sanitization is present:
>
> [   22.414170] lkdtm: Performing direct entry READ_AFTER_FREE
> [   22.415124] lkdtm: Value in memory before free: 12345678
> [   22.415900] lkdtm: Attempting to read from freed memory
> [   22.416394] lkdtm: Successfully read value: 12345678
>
> with sanitization:
>
> [   25.874585] lkdtm: Performing direct entry READ_AFTER_FREE
> [   25.875527] lkdtm: Value in memory before free: 12345678
> [   25.876382] lkdtm: Attempting to read from freed memory
> [   25.876900] general protection fault: 0000 [#1] SMP
>
> Signed-off-by: Laura Abbott <labbott@xxxxxxxxxxxxxxxxx>

Excellent! Could you mention in the changelog which CONFIG (or runtime
values) will change the lkdtm test? (I thought there was a poisoning
style that would result in a zero-read instead of a GP?)

-Kees

> ---
> I split this out from the previous series
> (http://article.gmane.org/gmane.linux.kernel.mm/143486) since
> that series is going to be going in more incrementally.
> Having the test in sooner than later will be helpful I think
>
> v2: Tweaked the output text to be clearer about what's going on.
> Switched to using the middle of an allocated block instead of the beginning.
> ---
>  drivers/misc/lkdtm.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
> index 11fdadc..24d0ac7 100644
> --- a/drivers/misc/lkdtm.c
> +++ b/drivers/misc/lkdtm.c
> @@ -92,6 +92,7 @@ enum ctype {
>         CT_UNALIGNED_LOAD_STORE_WRITE,
>         CT_OVERWRITE_ALLOCATION,
>         CT_WRITE_AFTER_FREE,
> +       CT_READ_AFTER_FREE,
>         CT_SOFTLOCKUP,
>         CT_HARDLOCKUP,
>         CT_SPINLOCKUP,
> @@ -129,6 +130,7 @@ static char* cp_type[] = {
>         "UNALIGNED_LOAD_STORE_WRITE",
>         "OVERWRITE_ALLOCATION",
>         "WRITE_AFTER_FREE",
> +       "READ_AFTER_FREE",
>         "SOFTLOCKUP",
>         "HARDLOCKUP",
>         "SPINLOCKUP",
> @@ -417,6 +419,38 @@ static void lkdtm_do_action(enum ctype which)
>                 memset(data, 0x78, len);
>                 break;
>         }
> +       case CT_READ_AFTER_FREE: {
> +               int **base;
> +               int *val, *tmp;
> +               size_t len = 1024;
> +               /*
> +                * The slub allocator uses the first word to store the free
> +                * pointer in some configurations. Use the middle of the
> +                * allocation to avoid running into the freelist
> +                */
> +               size_t offset = (len/sizeof(int *))/2;
> +
> +               base = kmalloc(len, GFP_KERNEL);
> +               if (!base)
> +                       return;
> +
> +               val = kmalloc(len, GFP_KERNEL);
> +               if (!val)
> +                       return;
> +
> +               *val = 0x12345678;
> +               pr_info("Value in memory before free: %x\n", *val);
> +
> +               base[offset] = val;
> +               kfree(base);
> +
> +               tmp = base[offset];
> +               pr_info("Attempting to read from freed memory");
> +               pr_info("Successfully read value: %x\n", *tmp);
> +
> +               kfree(val);
> +               break;
> +       }
>         case CT_SOFTLOCKUP:
>                 preempt_disable();
>                 for (;;)
> --
> 2.5.0
>



-- 
Kees Cook
Chrome OS & Brillo Security