[PATCH v3] lkdtm: fix memory copy size for WRITE_KERN

[PATCH v3] lkdtm: fix memory copy size for WRITE_KERN

[Candle Sun]

From: Candle Sun <candle.sun@unisoc.com>

Though do_overwritten() follows do_nothing() in source code, the final
memory address order is determined by the compiler. We can't always
assume address of do_overwritten() is bigger than do_nothing(). At least
the Clang we are using places do_overwritten() before do_nothing() in the
object. This causes the copy size in lkdtm_WRITE_KERN() is *really* big
and WRITE_KERN test on ARM32 arch will fail.

Get absolute value of the address subtraction for memcpy() size.

Signed-off-by: Candle Sun <candle.sun@unisoc.com>
---
Changes in v3:
- Clean some typo.
- Remove one comment line.
Changes in v2:
- Use abs() in place of address comparison.
---
 drivers/misc/lkdtm/perms.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 2dede2ef658f..c9227e08f97f 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -31,13 +31,12 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
-static void do_nothing(void)
+static noinline void do_nothing(void)
 {
 	return;
 }
 
-/* Must immediately follow do_nothing for size calculuations to work out. */
-static void do_overwritten(void)
+static noinline void do_overwritten(void)
 {
 	pr_info("do_overwritten wasn't overwritten!\n");
 	return;
@@ -113,7 +112,7 @@ void lkdtm_WRITE_KERN(void)
 	size_t size;
 	volatile unsigned char *ptr;
 
-	size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
+	size = (size_t)abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing);
 	ptr = (unsigned char *)do_overwritten;
 
 	pr_info("attempting bad %zu byte write at %px\n", size, ptr);
-- 
2.17.0

Re: [PATCH v3] lkdtm: fix memory copy size for WRITE_KERN

[Nick Desaulniers]

On Wed, Jan 27, 2021 at 10:58 PM Candle Sun <candlesea@gmail.com> wrote:
>
> From: Candle Sun <candle.sun@unisoc.com>
>
> Though do_overwritten() follows do_nothing() in source code, the final
> memory address order is determined by the compiler. We can't always
> assume address of do_overwritten() is bigger than do_nothing(). At least
> the Clang we are using places do_overwritten() before do_nothing() in the
> object. This causes the copy size in lkdtm_WRITE_KERN() is *really* big
> and WRITE_KERN test on ARM32 arch will fail.
>
> Get absolute value of the address subtraction for memcpy() size.

LGTM, thanks Candle!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Signed-off-by: Candle Sun <candle.sun@unisoc.com>
> ---
> Changes in v3:
> - Clean some typo.
> - Remove one comment line.
> Changes in v2:
> - Use abs() in place of address comparison.
> ---
>  drivers/misc/lkdtm/perms.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 2dede2ef658f..c9227e08f97f 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -31,13 +31,12 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
>   * This just returns to the caller. It is designed to be copied into
>   * non-executable memory regions.
>   */
> -static void do_nothing(void)
> +static noinline void do_nothing(void)
>  {
>         return;
>  }
>
> -/* Must immediately follow do_nothing for size calculuations to work out. */
> -static void do_overwritten(void)
> +static noinline void do_overwritten(void)
>  {
>         pr_info("do_overwritten wasn't overwritten!\n");
>         return;
> @@ -113,7 +112,7 @@ void lkdtm_WRITE_KERN(void)
>         size_t size;
>         volatile unsigned char *ptr;
>
> -       size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
> +       size = (size_t)abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing);
>         ptr = (unsigned char *)do_overwritten;
>
>         pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> --
> 2.17.0
>


-- 
Thanks,
~Nick Desaulniers