userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas

After the VMA to register the uffd onto is found, check that it has
VM_MAYWRITE set before allowing registration.  This way we inherit all
common code checks before allowing to fill file holes in shmem and
hugetlbfs with UFFDIO_COPY.

The userfaultfd memory model is not applicable for readonly files unless
it's a MAP_PRIVATE.

Link: http://lkml.kernel.org/r/20181126173452.26955-4-aarcange@redhat.com
Fixes: ff62a3421044 ("hugetlb: implement memfd sealing")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Hugh Dickins <hughd@google.com>
Reported-by: Jann Horn <jannh@google.com>
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Cc: <stable@vger.kernel.org>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	int ret;
	struct uffdio_range uffdio_wake;
	struct userfaultfd_wake_range range;
	const void __user *buf = (void __user *)arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
		goto out;

	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
	if (ret)
		goto out;

	range.start = uffdio_wake.start;
	range.len = uffdio_wake.len;

	/*
	 * len == 0 means wake all and we don't want to wake all here,
	 * so check it again to be sure.
	 */
	VM_BUG_ON(!range.len);

	wake_userfault(ctx, &range);
	ret = 0;

out:
	return ret;
}

static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	int ret;
	struct uffdio_range uffdio_wake;
	struct userfaultfd_wake_range range;
	const void __user *buf = (void __user *)arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
		goto out;

	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
	if (ret)
		goto out;

	range.start = uffdio_wake.start;
	range.len = uffdio_wake.len;

	/*
	 * len == 0 means wake all and we don't want to wake all here,
	 * so check it again to be sure.
	 */
	VM_BUG_ON(!range.len);

	wake_userfault(ctx, &range);
	ret = 0;

out:
	return ret;
}
