Re: [PATCH v6 24/41] mm: Don't allow write GUPs to shadow stack memory

[David Hildenbrand]

On 18.02.23 22:14, Rick Edgecombe wrote:
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which requires some core mm changes to function
> properly.
>
> Shadow stack memory is writable only in very specific, controlled ways.
> However, since it is writable, the kernel treats it as such. As a result
> there remain many ways for userspace to trigger the kernel to write to
> shadow stack's via get_user_pages(, FOLL_WRITE) operations. To make this a
> little less exposed, block writable GUPs for shadow stack VMAs.
>
> Still allow FOLL_FORCE to write through shadow stack protections, as it
> does for read-only protections.
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>
> ---
> v3:
>   - Add comment in __pte_access_permitted() (Dave)
>   - Remove unneeded shadow stack specific check in
>     __pte_access_permitted() (Jann)
> ---
>   arch/x86/include/asm/pgtable.h | 5 +++++
>   mm/gup.c                       | 2 +-
>   2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 6b7106457bfb..20d0df494269 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1641,6 +1641,11 @@ static inline bool __pte_access_permitted(unsigned long pteval, bool write)
>   {
>   	unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
>   
> +	/*
> +	 * Write=0,Dirty=1 PTEs are shadow stack, which the kernel
> +	 * shouldn't generally allow access to, but since they
> +	 * are already Write=0, the below logic covers both cases.
> +	 */
>   	if (write)
>   		need_pte_bits |= _PAGE_RW;

So, GUP fast will always fail when writing ...

>   
> diff --git a/mm/gup.c b/mm/gup.c
> index f45a3a5be53a..bfd33d9edb89 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -982,7 +982,7 @@ 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;
>   			/* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */

and ordinary GUP without FOLL_FORCE.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb