Re: [PATCH] mm: fix double page fault on arm64 if PTE_AF is cleared

Catalin Marinas
2019-09-04T13:49:31Z

On Wed, 4 Sep 2019 at 01:59, Jia He <justin.he@arm.com> wrote:
> @@ -2152,20 +2153,30 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
> */
> if (unlikely(!src)) {
> void *kaddr = kmap_atomic(dst);
> - void __user *uaddr = (void __user *)(va & PAGE_MASK);
> + void __user *uaddr = (void __user *)(vmf->address & PAGE_MASK);
> + pte_t entry;
>
> /*
> * This really shouldn't fail, because the page is there
> * in the page tables. But it might just be unreadable,
> * in which case we just give up and fill the result with
> - * zeroes.
> + * zeroes. If PTE_AF is cleared on arm64, it might
> + * cause double page fault here. so makes pte young here
> */
> + if (!pte_young(vmf->orig_pte)) {
> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vmf->vma, vmf->address,
> + vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE))

I think you need to pass dirty = 0 to ptep_set_access_flags() rather
than the vmf->flags & FAULT_FLAG_WRITE. This is copying from the user
address into a kernel mapping and the fault you want to prevent is a
read access on uaddr via __copy_from_user_inatomic(). The pte will be
made writable in the wp_page_copy() function.

--
Catalin