Subject: Re: [PATCH 06/11] lkdtm: disable set_fs-based tests for !CONFIG_SET_FS

From: Kees Cook <[email-masked]>
Date: Tue, 18 Aug 2020 12:32:36 -0700

On Mon, Aug 17, 2020 at 09:32:07AM +0200, Christoph Hellwig wrote:
> Once we can't manipulate the address limit, we also can't test what
> happens when the manipulation is abused.
> 
> Signed-off-by: Christoph Hellwig <[email-masked]>
> ---
>  drivers/misc/lkdtm/bugs.c     | 2 ++
>  drivers/misc/lkdtm/core.c     | 4 ++++
>  drivers/misc/lkdtm/usercopy.c | 2 ++
>  3 files changed, 8 insertions(+)
> 
> diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
> index 4dfbfd51bdf774..66f1800b1cb82d 100644
> --- a/drivers/misc/lkdtm/bugs.c
> +++ b/drivers/misc/lkdtm/bugs.c
> @@ -312,6 +312,7 @@ void lkdtm_CORRUPT_LIST_DEL(void)
>  		pr_err("list_del() corruption not detected!\n");
>  }
>  
> +#ifdef CONFIG_SET_FS
>  /* Test if unbalanced set_fs(KERNEL_DS)/set_fs(USER_DS) check exists. */
>  void lkdtm_CORRUPT_USER_DS(void)
>  {
> @@ -321,6 +322,7 @@ void lkdtm_CORRUPT_USER_DS(void)
>  	/* Make sure we do not keep running with a KERNEL_DS! */
>  	force_sig(SIGKILL);
>  }
> +#endif

Please let the test defined, but it should XFAIL with a message about
the CONFIG (see similar ifdefs in lkdtm).

>  /* Test that VMAP_STACK is actually allocating with a leading guard page */
>  void lkdtm_STACK_GUARD_PAGE_LEADING(void)
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index a5e344df916632..aae08b33a7ee2a 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -112,7 +112,9 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(CORRUPT_STACK_STRONG),
>  	CRASHTYPE(CORRUPT_LIST_ADD),
>  	CRASHTYPE(CORRUPT_LIST_DEL),
> +#ifdef CONFIG_SET_FS
>  	CRASHTYPE(CORRUPT_USER_DS),
> +#endif
>  	CRASHTYPE(STACK_GUARD_PAGE_LEADING),
>  	CRASHTYPE(STACK_GUARD_PAGE_TRAILING),
>  	CRASHTYPE(UNSET_SMEP),
> @@ -172,7 +174,9 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
>  	CRASHTYPE(USERCOPY_STACK_BEYOND),
>  	CRASHTYPE(USERCOPY_KERNEL),
> +#ifdef CONFIG_SET_FS
>  	CRASHTYPE(USERCOPY_KERNEL_DS),
> +#endif
>  	CRASHTYPE(STACKLEAK_ERASING),
>  	CRASHTYPE(CFI_FORWARD_PROTO),

Then none of these are needed.

>  #ifdef CONFIG_X86_32

Hmpf, this ifdef was missed in ae56942c1474 ("lkdtm: Make arch-specific
tests always available"). I will fix that.

> diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
> index b833367a45d053..4b632fe79ab6bb 100644
> --- a/drivers/misc/lkdtm/usercopy.c
> +++ b/drivers/misc/lkdtm/usercopy.c
> @@ -325,6 +325,7 @@ void lkdtm_USERCOPY_KERNEL(void)
>  	vm_munmap(user_addr, PAGE_SIZE);
>  }
>  
> +#ifdef CONFIG_SET_FS
>  void lkdtm_USERCOPY_KERNEL_DS(void)
>  {
>  	char __user *user_ptr =
> @@ -339,6 +340,7 @@ void lkdtm_USERCOPY_KERNEL_DS(void)
>  		pr_err("copy_to_user() to noncanonical address succeeded!?\n");
>  	set_fs(old_fs);
>  }
> +#endif

(Same here, please.)

>  
>  void __init lkdtm_usercopy_init(void)
>  {
> -- 
> 2.28.0
> 

-- 
Kees Cook