Re: [PATCH v1 08/10] lkdtm: Really write into kernel text in WRITE_KERN

Kees Cook
2021-10-13T07:05:26Z

On Mon, Oct 11, 2021 at 05:25:35PM +0200, Christophe Leroy wrote:
> WRITE_KERN is supposed to overwrite some kernel text, namely
> do_overwritten() function.
>
> But at the time being it overwrites do_overwritten() function
> descriptor, not function text.
>
> Fix it by dereferencing the function descriptor to obtain
> function text pointer.
>
> And make do_overwritten() noinline so that it is really
> do_overwritten() which is called by lkdtm_WRITE_KERN().
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> drivers/misc/lkdtm/perms.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 60b3b2fe929d..442d60ed25ef 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -5,6 +5,7 @@
> * even non-readable regions.
> */
> #include "lkdtm.h"
> +#include <linux/kallsyms.h>

Why not #include <asm/sections.h> instead here?

> #include <linux/slab.h>
> #include <linux/vmalloc.h>
> #include <linux/mman.h>
> @@ -37,7 +38,7 @@ static noinline void do_nothing(void)
> }
>
> /* 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;
> @@ -113,8 +114,9 @@ void lkdtm_WRITE_KERN(void)
> size_t size;
> volatile unsigned char *ptr;
>
> - size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
> - ptr = (unsigned char *)do_overwritten;
> + size = (unsigned long)dereference_symbol_descriptor(do_overwritten) -
> + (unsigned long)dereference_symbol_descriptor(do_nothing);
> + ptr = dereference_symbol_descriptor(do_overwritten);

But otherwise, yup, I expect there will be a bunch of things like this
to clean up in LKDTM. :| Sorry about that!

Acked-by: Kees Cook <keescook@chromium.org>

>
> pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> memcpy((void *)ptr, (unsigned char *)do_nothing, size);
> --
> 2.31.1
>

--
Kees Cook