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

["Justin He (Arm Technology China)" <Justin.He@arm.com>]

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@arm.com>
> Sent: 2019年9月4日 21:49
> To: Justin He (Arm Technology China) <Justin.He@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>; Matthew Wilcox
> <willy@infradead.org>; Jérôme Glisse <jglisse@redhat.com>; Ralph
> Campbell <rcampbell@nvidia.com>; Jason Gunthorpe <jgg@ziepe.ca>;
> Peter Zijlstra <peterz@infradead.org>; Dave Airlie <airlied@redhat.com>;
> Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>; Thomas Hellstrom
> <thellstrom@vmware.com>; Souptick Joarder <jrdr.linux@gmail.com>;
> linux-mm <linux-mm@kvack.org>; Linux Kernel Mailing List <linux-
> kernel@vger.kernel.org>
> Subject: Re: [PATCH] mm: fix double page fault on arm64 if PTE_AF is
> cleared
>
> 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.

Ok, thanks

--
Cheers,
Justin (Jia He)


>
> --
> Catalin
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