Subject: Re: [PATCH v2 12/13] lkdtm: Fix execute_[user]_location()

From: Kees Cook <[email-masked]>
Date: Fri, 15 Oct 2021 14:31:33 -0700

On Thu, Oct 14, 2021 at 07:50:01AM +0200, Christophe Leroy wrote:
> execute_location() and execute_user_location() intent
> to copy do_nothing() text and execute it at a new location.
> However, at the time being it doesn't copy do_nothing() function
> but do_nothing() function descriptor which still points to the
> original text. So at the end it still executes do_nothing() at
> its original location allthough using a copied function descriptor.
> 
> So, fix that by really copying do_nothing() text and build a new
> function descriptor by copying do_nothing() function descriptor and
> updating the target address with the new location.
> 
> Also fix the displayed addresses by dereferencing do_nothing()
> function descriptor.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  drivers/misc/lkdtm/perms.c     | 25 +++++++++++++++++++++----
>  include/asm-generic/sections.h |  5 +++++
>  2 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 5266dc28df6e..96b3ebfcb8ed 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,19 +44,32 @@ static noinline void do_overwritten(void)
>  	return;
>  }
>  
> +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> +{
> +	memcpy(fdesc, do_nothing, sizeof(*fdesc));
> +	fdesc->addr = (unsigned long)dst;
> +	barrier();
> +
> +	return fdesc;
> +}

How about collapsing the "have_function_descriptors()" check into
setup_function_descriptor()?

static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
{
	if (__is_defined(HAVE_FUNCTION_DESCRIPTORS)) {
		memcpy(fdesc, do_nothing, sizeof(*fdesc));
		fdesc->addr = (unsigned long)dst;
		barrier();
		return fdesc;
	} else {
		return dst;
	}
}

> +
>  static noinline void execute_location(void *dst, bool write)
>  {
>  	void (*func)(void) = dst;
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>  
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>  	do_nothing();
>  
>  	if (write == CODE_WRITE) {
> -		memcpy(dst, do_nothing, EXEC_SIZE);
> +		memcpy(dst, do_nothing_text, EXEC_SIZE);
>  		flush_icache_range((unsigned long)dst,
>  				   (unsigned long)dst + EXEC_SIZE);
>  	}
>  	pr_info("attempting bad execution at %px\n", func);
> +	if (have_function_descriptors())
> +		func = setup_function_descriptor(&fdesc, dst);
>  	func();
>  	pr_err("FAIL: func returned\n");
>  }
> @@ -67,15 +80,19 @@ static void execute_user_location(void *dst)
>  
>  	/* Intentionally crossing kernel/user memory boundary. */
>  	void (*func)(void) = dst;
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>  
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>  	do_nothing();
>  
> -	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> +	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
>  				   EXEC_SIZE, FOLL_WRITE);
>  	if (copied < EXEC_SIZE)
>  		return;
>  	pr_info("attempting bad execution at %px\n", func);
> +	if (have_function_descriptors())
> +		func = setup_function_descriptor(&fdesc, dst);
>  	func();
>  	pr_err("FAIL: func returned\n");
>  }

> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index 76163883c6ff..d225318538bd 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -70,6 +70,11 @@ typedef struct {
>  } func_desc_t;
>  #endif
>  
> +static inline bool have_function_descriptors(void)
> +{
> +	return __is_defined(HAVE_FUNCTION_DESCRIPTORS);
> +}
> +
>  /* random extra sections (if any).  Override
>   * in asm/sections.h */
>  #ifndef arch_is_kernel_text

This hunk seems like it should live in a separate patch.

-- 
Kees Cook