Re: [PATCH v1 3/4] mm: Protect kernel pgtables in apply_to_pte_range()

["Nicholas Piggin" <npiggin@gmail.com>]

On Tue Apr 8, 2025 at 1:11 AM AEST, Alexander Gordeev wrote:
> The lazy MMU mode can only be entered and left under the protection
> of the page table locks for all page tables which may be modified.
> Yet, when it comes to kernel mappings apply_to_pte_range() does not
> take any locks. That does not conform arch_enter|leave_lazy_mmu_mode()
> semantics and could potentially lead to re-schedulling a process while
> in lazy MMU mode or racing on a kernel page table updates.
>
> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
> ---
>  mm/kasan/shadow.c | 7 ++-----
>  mm/memory.c       | 5 ++++-
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index edfa77959474..6531a7aa8562 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -308,14 +308,14 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>  	__memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
>  	pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
>  
> -	spin_lock(&init_mm.page_table_lock);
>  	if (likely(pte_none(ptep_get(ptep)))) {
>  		set_pte_at(&init_mm, addr, ptep, pte);
>  		page = 0;
>  	}
> -	spin_unlock(&init_mm.page_table_lock);
> +
>  	if (page)
>  		free_page(page);
> +
>  	return 0;
>  }
>  

kasan_populate_vmalloc_pte() is really the only thing that
takes the ptl in the apply_to_page_range fn()... Looks like
you may be right. I wonder why they do and nobody else? Just
luck?

Seems okay.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

> @@ -401,13 +401,10 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>  
>  	page = (unsigned long)__va(pte_pfn(ptep_get(ptep)) << PAGE_SHIFT);
>  
> -	spin_lock(&init_mm.page_table_lock);
> -
>  	if (likely(!pte_none(ptep_get(ptep)))) {
>  		pte_clear(&init_mm, addr, ptep);
>  		free_page(page);
>  	}
> -	spin_unlock(&init_mm.page_table_lock);
>  
>  	return 0;
>  }
> diff --git a/mm/memory.c b/mm/memory.c
> index f0201c8ec1ce..1f3727104e99 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2926,6 +2926,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  			pte = pte_offset_kernel(pmd, addr);
>  		if (!pte)
>  			return err;
> +		spin_lock(&init_mm.page_table_lock);
>  	} else {
>  		if (create)
>  			pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
> @@ -2951,7 +2952,9 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  
>  	arch_leave_lazy_mmu_mode();
>  
> -	if (mm != &init_mm)
> +	if (mm == &init_mm)
> +		spin_unlock(&init_mm.page_table_lock);
> +	else
>  		pte_unmap_unlock(mapped_pte, ptl);
>  
>  	*mask |= PGTBL_PTE_MODIFIED;