Re: [PATCH v2 20/26] userfaultfd: wp: support write protection for userfault vma range

Mike Rapoport
2019-02-26T06:44:04Z

On Tue, Feb 26, 2019 at 02:06:27PM +0800, Peter Xu wrote:
> On Mon, Feb 25, 2019 at 10:52:34PM +0200, Mike Rapoport wrote:
> > On Tue, Feb 12, 2019 at 10:56:26AM +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: 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>
> > > [peterx:
> > > - use the helper to find VMA;
> > > - return -ENOENT if not found to match mcopy case;
> > > - use the new MM_CP_UFFD_WP* flags for change_protection
> > > - check against mmap_changing for failures]
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > > include/linux/userfaultfd_k.h | 3 ++
> > > mm/userfaultfd.c | 54 +++++++++++++++++++++++++++++++++++
> > > 2 files changed, 57 insertions(+)
> > >
> > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > > index 765ce884cec0..8f6e6ed544fb 100644
> > > --- a/include/linux/userfaultfd_k.h
> > > +++ b/include/linux/userfaultfd_k.h
> > > @@ -39,6 +39,9 @@ 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, bool *mmap_changing);
> > >
> > > /* 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 fefa81c301b7..529d180bb4d7 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -639,3 +639,57 @@ ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
> > > {
> > > return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing, 0);
> > > }
> > > +
> > > +int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> > > + unsigned long len, bool enable_wp, bool *mmap_changing)
> > > +{
> > > + 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);
> >
> > I'd replace these BUG_ON()s with
> >
> > if (WARN_ON())
> > return -EINVAL;
>
> I believe BUG_ON() is used because these parameters should have been
> checked in userfaultfd_writeprotect() already by the common
> validate_range() even before calling mwriteprotect_range(). So I'm
> fine with the WARN_ON() approach but I'd slightly prefer to simply
> keep the patch as is to keep Jerome's r-b if you won't disagree. :)

Right, userfaultfd_writeprotect() should check these parameters and if it
didn't it was a bug indeed. But still, it's not severe enough to crash the
kernel.

I hope Jerome wouldn't mind to keep his r-b with s/BUG_ON/WARN_ON ;-)

With this change you can also add

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> Thanks,
>
> --
> Peter Xu
>

--
Sincerely yours,
Mike.