Subject: Re: [PATCH v2 13/13] lkdtm: Add a test for function descriptors protection

From: Kees Cook <[email-masked]>
Date: Fri, 15 Oct 2021 14:35:44 -0700

On Thu, Oct 14, 2021 at 07:50:02AM +0200, Christophe Leroy wrote:
> Add WRITE_OPD to check that you can't modify function
> descriptors.
> 
> Gives the following result when function descriptors are
> not protected:
> 
> 	lkdtm: Performing direct entry WRITE_OPD
> 	lkdtm: attempting bad 16 bytes write at c00000000269b358
> 	lkdtm: FAIL: survived bad write
> 	lkdtm: do_nothing was hijacked!
> 
> Looks like a standard compiler barrier(); is not enough to force
> GCC to use the modified function descriptor. Add to add a fake empty
> inline assembly to force GCC to reload the function descriptor.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  drivers/misc/lkdtm/core.c  |  1 +
>  drivers/misc/lkdtm/lkdtm.h |  1 +
>  drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index fe6fd34b8caf..de092aa03b5d 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(WRITE_RO),
>  	CRASHTYPE(WRITE_RO_AFTER_INIT),
>  	CRASHTYPE(WRITE_KERN),
> +	CRASHTYPE(WRITE_OPD),
>  	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index c212a253edde..188bd0fd6575 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
>  void lkdtm_WRITE_RO(void);
>  void lkdtm_WRITE_RO_AFTER_INIT(void);
>  void lkdtm_WRITE_KERN(void);
> +void lkdtm_WRITE_OPD(void);
>  void lkdtm_EXEC_DATA(void);
>  void lkdtm_EXEC_STACK(void);
>  void lkdtm_EXEC_KMALLOC(void);
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 96b3ebfcb8ed..3870bc82d40d 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
>  	return;
>  }
>  
> +static noinline void do_almost_nothing(void)
> +{
> +	pr_info("do_nothing was hijacked!\n");
> +}
> +
>  static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>  {
>  	memcpy(fdesc, do_nothing, sizeof(*fdesc));
> @@ -143,6 +148,23 @@ void lkdtm_WRITE_KERN(void)
>  	do_overwritten();
>  }
>  
> +void lkdtm_WRITE_OPD(void)
> +{
> +	size_t size = sizeof(func_desc_t);
> +	void (*func)(void) = do_nothing;
> +
> +	if (!have_function_descriptors()) {
> +		pr_info("Platform doesn't have function descriptors.\n");

This should be more explicit ('xfail'):

	pr_info("XFAIL: platform doesn't use function descriptors.\n");

> +		return;
> +	}
> +	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
> +	memcpy(do_nothing, do_almost_nothing, size);
> +	pr_err("FAIL: survived bad write\n");
> +
> +	asm("" : "=m"(func));

Since this is a descriptor, I assume no icache flush is needed. Are
function descriptors strictly dcache? (Is anything besides just a
barrier needed?)

> +	func();
> +}
> +
>  void lkdtm_EXEC_DATA(void)
>  {
>  	execute_location(data_area, CODE_WRITE);
> -- 
> 2.31.1
> 

-- 
Kees Cook