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

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

Hi Catalin

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@arm.com>
> Sent: 2019年9月4日 22:22
> To: Anshuman Khandual <Anshuman.Khandual@arm.com>
> Cc: Justin He (Arm Technology China) <Justin.He@arm.com>; 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 04:20, Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
> > On 09/04/2019 06:28 AM, Jia He 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))
> > > +                             update_mmu_cache(vmf->vma, vmf->address,
> > > +                                             vmf->pte);
> > > +             }
> > > +
> > >               if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> >
> > Should not page fault be disabled when doing this ?
>
> Page faults are already disabled by the kmap_atomic(). But that only
> means that you don't deadlock trying to take the mmap_sem again.
>
> > Ideally it should
> > have also called access_ok() on the user address range first.
>
> Not necessary, we've already got a vma and the access to the vma checked.
>
> > The point
> > is that the caller of __copy_from_user_inatomic() must make sure that
> > there cannot be any page fault while doing the actual copy.
>
> When you copy from a user address, in general that's not guaranteed,
> more of a best effort.
>
> > But also it
> > should be done in generic way, something like in access_ok(). The current
> > proposal here seems very specific to arm64 case.
>
> The commit log didn't explain the problem properly. On arm64 without
> hardware Access Flag, copying from user will fail because the pte is
> old and cannot be marked young. So we always end up with zeroed page
> after fork() + CoW for pfn mappings.
>

Ok I will update it, thanks

--
Cheers,
Justin (Jia He)


> --
> Catalin
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