Re: [RFC][PATCH 7/7] lkdtm: Add READ_AFTER_FREE test

[Kees Cook <keescook@chromium.org>]

On Mon, Dec 21, 2015 at 7:40 PM, Laura Abbott <laura@labbott.name> wrote:
>
> In a similar manner to WRITE_AFTER_FREE, add a READ_AFTER_FREE
> test to test free poisoning features. Sample output when
> no poison is present:
>
> [   20.222501] lkdtm: Performing direct entry READ_AFTER_FREE
> [   20.226163] lkdtm: Freed val: 12345678
>
> with poison:
>
> [   24.203748] lkdtm: Performing direct entry READ_AFTER_FREE
> [   24.207261] general protection fault: 0000 [#1] SMP
> [   24.208193] Modules linked in:
> [   24.208193] CPU: 0 PID: 866 Comm: sh Not tainted 4.4.0-rc5-work+ #108
>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Laura Abbott <laura@labbott.name>
> ---
>  drivers/misc/lkdtm.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
> index 11fdadc..c641fb7 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 +419,33 @@ static void lkdtm_do_action(enum ctype which)
>                 memset(data, 0x78, len);
>                 break;
>         }
> +       case CT_READ_AFTER_FREE: {
> +               int **base;
> +               int *val, *tmp;
> +
> +               base = kmalloc(1024, GFP_KERNEL);
> +               if (!base)
> +                       return;
> +
> +               val = kmalloc(1024, GFP_KERNEL);
> +               if (!val)
> +                       return;

For both of these test failure return, I think there should be a
pr_warn too (see CT_EXEC_USERSPACE).

> +
> +               *val = 0x12345678;
> +
> +               /*
> +                * Don't just use the first entry since that's where the
> +                * freelist goes for the slab allocator
> +                */
> +               base[1] = val;

Maybe just aim at the middle, in case allocator freelist tracking ever
grows? base[1024/sizeof(int)/2] or something?

> +               kfree(base);
> +
> +               tmp = base[1];
> +               pr_info("Freed val: %x\n", *tmp);

Instead of depending on the deref to fail, maybe just use a simple
BUG_ON to test that the value did actually change? Or, change the
pr_info to "Failed to Oops when reading freed value: ..." just to be
slightly more verbose about what failed?

> +
> +               kfree(val);
> +               break;
> +       }
>         case CT_SOFTLOCKUP:
>                 preempt_disable();
>                 for (;;)
> --
> 2.5.0
>

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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