Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range

Peter Xu
2019-01-22T08:55:38Z

On Mon, Jan 21, 2019 at 12:20:35PM +0200, Mike Rapoport wrote:
> On Mon, Jan 21, 2019 at 03:57:04PM +0800, Peter Xu wrote:
> > From: Shaohua Li <shli@fb.com>
> >
> > Add API to enable/disable writeprotect a vma range. Unlike mprotect,
> > this doesn't split/merge vmas.
> >
> > Cc: Andrea Arcangeli <aarcange@redhat.com>
> > Cc: Pavel Emelyanov <xemul@parallels.com>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Kirill A. Shutemov <kirill@shutemov.name>
> > Cc: Mel Gorman <mgorman@suse.de>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Shaohua Li <shli@fb.com>
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > include/linux/userfaultfd_k.h | 2 ++
> > mm/userfaultfd.c | 52 +++++++++++++++++++++++++++++++++++
> > 2 files changed, 54 insertions(+)
> >
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index 38f748e7186e..e82f3156f4e9 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -37,6 +37,8 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
> > unsigned long dst_start,
> > unsigned long len,
> > bool *mmap_changing);
> > +extern int mwriteprotect_range(struct mm_struct *dst_mm,
> > + unsigned long start, unsigned long len, bool enable_wp);
> >
> > /* mm helpers */
> > static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 458acda96f20..c38903f501c7 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -615,3 +615,55 @@ ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
> > {
> > return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing);
> > }
> > +
> > +int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> > + unsigned long len, bool enable_wp)
> > +{
> > + struct vm_area_struct *dst_vma;
> > + pgprot_t newprot;
> > + int err;
> > +
> > + /*
> > + * Sanitize the command parameters:
> > + */
> > + BUG_ON(start & ~PAGE_MASK);
> > + BUG_ON(len & ~PAGE_MASK);
> > +
> > + /* Does the address range wrap, or is the span zero-sized? */
> > + BUG_ON(start + len <= start);
> > +
> > + down_read(&dst_mm->mmap_sem);
> > +
> > + /*
> > + * Make sure the vma is not shared, that the dst range is
> > + * both valid and fully within a single existing vma.
> > + */
> > + err = -EINVAL;
>
> In non-cooperative mode, there can be a race between VM layout changes and
> mcopy_atomic [1]. I believe the same races are possible here, so can we
> please make err = -ENOENT for consistency with mcopy?

Sure.

>
> > + dst_vma = find_vma(dst_mm, start);
> > + if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
> > + goto out_unlock;
> > + if (start < dst_vma->vm_start ||
> > + start + len > dst_vma->vm_end)
> > + goto out_unlock;
> > +
> > + if (!dst_vma->vm_userfaultfd_ctx.ctx)
> > + goto out_unlock;
> > + if (!userfaultfd_wp(dst_vma))
> > + goto out_unlock;
> > +
> > + if (!vma_is_anonymous(dst_vma))
> > + goto out_unlock;
>
> The sanity checks here seem to repeat those in mcopy_atomic(). I'd suggest
> splitting them out to a helper function.

It's a good suggestion. Thanks!

>
> > + if (enable_wp)
> > + newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
> > + else
> > + newprot = vm_get_page_prot(dst_vma->vm_flags);
> > +
> > + change_protection(dst_vma, start, start + len, newprot,
> > + !enable_wp, 0);
> > +
> > + err = 0;
> > +out_unlock:
> > + up_read(&dst_mm->mmap_sem);
> > + return err;
> > +}
> > --
> > 2.17.1
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=27d02568f529e908399514dfbee8ee43bdfd5299
>
> --
> Sincerely yours,
> Mike.
>

--
Peter Xu