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>
void mremap_userfaultfd_prep(struct vm_area_struct *vma,
			     struct vm_userfaultfd_ctx *vm_ctx)
{
	struct userfaultfd_ctx *ctx;

	ctx = vma->vm_userfaultfd_ctx.ctx;
	if (ctx && (ctx->features & UFFD_FEATURE_EVENT_REMAP)) {
		vm_ctx->ctx = ctx;
		userfaultfd_ctx_get(ctx);
		WRITE_ONCE(ctx->mmap_changing, true);
	}
}

void mremap_userfaultfd_prep(struct vm_area_struct *vma,
			     struct vm_userfaultfd_ctx *vm_ctx)
{
	struct userfaultfd_ctx *ctx;

	ctx = vma->vm_userfaultfd_ctx.ctx;
	if (ctx && (ctx->features & UFFD_FEATURE_EVENT_REMAP)) {
		vm_ctx->ctx = ctx;
		userfaultfd_ctx_get(ctx);
		WRITE_ONCE(ctx->mmap_changing, true);
	}
}
