Re: [PATCH v24 20/30] mm/mprotect: Exclude shadow stack from preserve_write

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Apr 01, 2021 at 03:10:54PM -0700, Yu-cheng Yu wrote:
> In change_pte_range(), when a PTE is changed for prot_numa, _PAGE_RW is
> preserved to avoid the additional write fault after the NUMA hinting fault.
> However, pte_write() now includes both normal writable and shadow stack
> (RW=0, Dirty=1) PTEs, but the latter does not have _PAGE_RW and has no need
> to preserve it.
> 
> Exclude shadow stack from preserve_write test, and apply the same change to
> change_huge_pmd().
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> v24:
> - Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().
> 
>  mm/huge_memory.c | 7 ++++++-
>  mm/mprotect.c    | 9 ++++++++-
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 65fc0aedd577..1d41138c4f74 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1812,12 +1812,17 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
>  	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
>  	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> +	bool shstk = is_shadow_stack_mapping(vma->vm_flags);
>  
>  	ptl = __pmd_trans_huge_lock(pmd, vma);
>  	if (!ptl)
>  		return 0;
>  
> -	preserve_write = prot_numa && pmd_write(*pmd);
> +	/*
> +	 * Preserve only normal writable huge PMD, but not shadow
> +	 * stack (RW=0, Dirty=1).
> +	 */
> +	preserve_write = prot_numa && pmd_write(*pmd) && !shstk;

New variable seems unnecessary. What about just:

	if (is_shadow_stack_mapping(vma->vm_flags))
		preserve_write = false;

?

>  	ret = 1;
>  
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index c1ce78d688b6..550448dc5ff1 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -75,7 +75,14 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  		oldpte = *pte;
>  		if (pte_present(oldpte)) {
>  			pte_t ptent;
> -			bool preserve_write = prot_numa && pte_write(oldpte);
> +			bool shstk = is_shadow_stack_mapping(vma->vm_flags);
> +			bool preserve_write;
> +
> +			/*
> +			 * Preserve only normal writable PTE, but not shadow
> +			 * stack (RW=0, Dirty=1).
> +			 */
> +			preserve_write = prot_numa && pte_write(oldpte) && !shstk;

Ditto.

>  
>  			/*
>  			 * Avoid trapping faults against the zero or KSM
> -- 
> 2.21.0
> 
> 

-- 
 Kirill A. Shutemov