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_api(struct userfaultfd_ctx *ctx,
			   unsigned long arg)
{
	struct uffdio_api uffdio_api;
	void __user *buf = (void __user *)arg;
	int ret;
	__u64 features;

	ret = -EINVAL;
	if (ctx->state != UFFD_STATE_WAIT_API)
		goto out;
	ret = -EFAULT;
	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
		goto out;
	features = uffdio_api.features;
	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
		memset(&uffdio_api, 0, sizeof(uffdio_api));
		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
			goto out;
		ret = -EINVAL;
		goto out;
	}
	/* report all available features and ioctls to userland */
	uffdio_api.features = UFFD_API_FEATURES;
	uffdio_api.ioctls = UFFD_API_IOCTLS;
	ret = -EFAULT;
	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
		goto out;
	ctx->state = UFFD_STATE_RUNNING;
	/* only enable the requested features for this uffd context */
	ctx->features = uffd_ctx_features(features);
	ret = 0;
out:
	return ret;
}

static int userfaultfd_api(struct userfaultfd_ctx *ctx,
			   unsigned long arg)
{
	struct uffdio_api uffdio_api;
	void __user *buf = (void __user *)arg;
	int ret;
	__u64 features;

	ret = -EINVAL;
	if (ctx->state != UFFD_STATE_WAIT_API)
		goto out;
	ret = -EFAULT;
	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
		goto out;
	features = uffdio_api.features;
	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
		memset(&uffdio_api, 0, sizeof(uffdio_api));
		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
			goto out;
		ret = -EINVAL;
		goto out;
	}
	/* report all available features and ioctls to userland */
	uffdio_api.features = UFFD_API_FEATURES;
	uffdio_api.ioctls = UFFD_API_IOCTLS;
	ret = -EFAULT;
	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
		goto out;
	ctx->state = UFFD_STATE_RUNNING;
	/* only enable the requested features for this uffd context */
	ctx->features = uffd_ctx_features(features);
	ret = 0;
out:
	return ret;
}
