Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

[Kees Cook]

On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
> [...]
> Still allow FOLL_FORCE to write through shadow stack protections, as it
> does for read-only protections.

As I asked in the cover letter: why do we need to add this for shstk? It
was a mistake for general memory. :P

> [...]
> diff --git a/mm/gup.c b/mm/gup.c
> index 5abdaf487460..56da98f3335c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1043,7 +1043,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>  		return -EFAULT;
>  
>  	if (write) {
> -		if (!(vm_flags & VM_WRITE)) {
> +		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
>  			if (!(gup_flags & FOLL_FORCE))
>  				return -EFAULT;
>  			/*

How about this instead:

  		return -EFAULT;
  
 	if (write) {
+		if (vm_flags & VM_SHADOW_STACK)
+			return -EFAULT;
 		if (!(vm_flags & VM_WRITE)) {
 			if (!(gup_flags & FOLL_FORCE))
 				return -EFAULT;


-- 
Kees Cook