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

From: Kees Cook <[email-masked]>
Date: Wed, 13 Oct 2021 00:05:26 -0700

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 <[email-masked]>
> ---
>  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 <[email-masked]>

>  
>  	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