Re: [PATCH v2 10/26] userfaultfd: wp: add UFFDIO_COPY_MODE_WP

Peter Xu
2019-02-26T05:10:02Z

On Mon, Feb 25, 2019 at 05:58:37PM +0200, Mike Rapoport wrote:
> On Tue, Feb 12, 2019 at 10:56:16AM +0800, Peter Xu wrote:
> > From: Andrea Arcangeli <aarcange@redhat.com>
> >
> > This allows UFFDIO_COPY to map pages wrprotected.
> write protected please :)

Sure!

> >
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
>
> Except for two additional nits below
>
> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
>
> > ---
> > fs/userfaultfd.c | 5 +++--
> > include/linux/userfaultfd_k.h | 2 +-
> > include/uapi/linux/userfaultfd.h | 11 +++++-----
> > mm/userfaultfd.c | 36 ++++++++++++++++++++++----------
> > 4 files changed, 35 insertions(+), 19 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index b397bc3b954d..3092885c9d2c 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -1683,11 +1683,12 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
> > ret = -EINVAL;
> > if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
> > goto out;
> > - if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
> > + if (uffdio_copy.mode & ~(UFFDIO_COPY_MODE_DONTWAKE|UFFDIO_COPY_MODE_WP))
> > goto out;
> > if (mmget_not_zero(ctx->mm)) {
> > ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
> > - uffdio_copy.len, &ctx->mmap_changing);
> > + uffdio_copy.len, &ctx->mmap_changing,
> > + uffdio_copy.mode);
> > mmput(ctx->mm);
> > } else {
> > return -ESRCH;
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index c6590c58ce28..765ce884cec0 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -34,7 +34,7 @@ extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
> >
> > extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
> > unsigned long src_start, unsigned long len,
> > - bool *mmap_changing);
> > + bool *mmap_changing, __u64 mode);
> > extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
> > unsigned long dst_start,
> > unsigned long len,
> > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > index 48f1a7c2f1f0..297cb044c03f 100644
> > --- a/include/uapi/linux/userfaultfd.h
> > +++ b/include/uapi/linux/userfaultfd.h
> > @@ -203,13 +203,14 @@ struct uffdio_copy {
> > __u64 dst;
> > __u64 src;
> > __u64 len;
> > +#define UFFDIO_COPY_MODE_DONTWAKE ((__u64)1<<0)
> > /*
> > - * There will be a wrprotection flag later that allows to map
> > - * pages wrprotected on the fly. And such a flag will be
> > - * available if the wrprotection ioctl are implemented for the
> > - * range according to the uffdio_register.ioctls.
> > + * UFFDIO_COPY_MODE_WP will map the page wrprotected on the
> > + * fly. UFFDIO_COPY_MODE_WP is available only if the
> > + * wrprotection ioctl are implemented for the range according
>
> ^ is

Will fix.

>
> > + * to the uffdio_register.ioctls.
> > */
> > -#define UFFDIO_COPY_MODE_DONTWAKE ((__u64)1<<0)
> > +#define UFFDIO_COPY_MODE_WP ((__u64)1<<1)
> > __u64 mode;
> >
> > /*
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index d59b5a73dfb3..73a208c5c1e7 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -25,7 +25,8 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
> > struct vm_area_struct *dst_vma,
> > unsigned long dst_addr,
> > unsigned long src_addr,
> > - struct page **pagep)
> > + struct page **pagep,
> > + bool wp_copy)
> > {
> > struct mem_cgroup *memcg;
> > pte_t _dst_pte, *dst_pte;
> > @@ -71,9 +72,9 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
> > if (mem_cgroup_try_charge(page, dst_mm, GFP_KERNEL, &memcg, false))
> > goto out_release;
> >
> > - _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
> > - if (dst_vma->vm_flags & VM_WRITE)
> > - _dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
> > + _dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
> > + if (dst_vma->vm_flags & VM_WRITE && !wp_copy)
> > + _dst_pte = pte_mkwrite(_dst_pte);
> >
> > dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> > if (dst_vma->vm_file) {
> > @@ -399,7 +400,8 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> > unsigned long dst_addr,
> > unsigned long src_addr,
> > struct page **page,
> > - bool zeropage)
> > + bool zeropage,
> > + bool wp_copy)
> > {
> > ssize_t err;
> >
> > @@ -416,11 +418,13 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> > if (!(dst_vma->vm_flags & VM_SHARED)) {
> > if (!zeropage)
> > err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
> > - dst_addr, src_addr, page);
> > + dst_addr, src_addr, page,
> > + wp_copy);
> > else
> > err = mfill_zeropage_pte(dst_mm, dst_pmd,
> > dst_vma, dst_addr);
> > } else {
> > + VM_WARN_ON(wp_copy); /* WP only available for anon */
> > if (!zeropage)
> > err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
> > dst_vma, dst_addr,
> > @@ -438,7 +442,8 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
> > unsigned long src_start,
> > unsigned long len,
> > bool zeropage,
> > - bool *mmap_changing)
> > + bool *mmap_changing,
> > + __u64 mode)
> > {
> > struct vm_area_struct *dst_vma;
> > ssize_t err;
> > @@ -446,6 +451,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
> > unsigned long src_addr, dst_addr;
> > long copied;
> > struct page *page;
> > + bool wp_copy;
> >
> > /*> * Sanitize the command parameters:
> > @@ -502,6 +508,14 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
> > dst_vma->vm_flags & VM_SHARED))
> > goto out_unlock;
> >
> > + /*
> > + * validate 'mode' now that we know the dst_vma: don't allow
> > + * a wrprotect copy if the userfaultfd didn't register as WP.
> > + */
> > + wp_copy = mode & UFFDIO_COPY_MODE_WP;
> > + if (wp_copy && !(dst_vma->vm_flags & VM_UFFD_WP))
> > + goto out_unlock;

[1]

> > +
> > /*
> > * If this is a HUGETLB vma, pass off to appropriate routine
> > */
>
> I think for hugetlb we should return an error if wp_copy==true.
> It might be worth adding wp_copy parameter to __mcopy_atomic_hugetlb() in
> advance and return the error from there, in a hope it will also support
> UFFD_WP some day :)

Now we should have failed even earlier if someone wants to register a
hugetlbfs VMA with UFFD_WP because now vma_can_userfault() only allows
anonymous memory for it:

static inline bool vma_can_userfault(struct vm_area_struct *vma,
unsigned long vm_flags)
{
/* FIXME: add WP support to hugetlbfs and shmem */
return vma_is_anonymous(vma) ||
((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
!(vm_flags & VM_UFFD_WP));
}

And, as long as a VMA is not tagged with UFFD_WP, the page copy will
fail with -EINVAL directly above at [1] when setting the wp_copy flag.
So IMHO we should have already covered the case.

Considering these, I would think we could simply postpone the changes
to __mcopy_atomic_hugetlb() until adding hugetlbfs support on uffd-wp.
Mike, what do you think?

Thanks!

--
Peter Xu