Re: [PATCH v15 06/11] mm: introduce memfd_secret system call to create "secret" memory areas

[Matthew Wilcox]

On Wed, Jan 20, 2021 at 08:06:07PM +0200, Mike Rapoport wrote:
> +static struct page *secretmem_alloc_page(gfp_t gfp)
> +{
> +	/*
> +	 * FIXME: use a cache of large pages to reduce the direct map
> +	 * fragmentation
> +	 */
> +	return alloc_page(gfp);
> +}
> +
> +static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> +{
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	pgoff_t offset = vmf->pgoff;
> +	unsigned long addr;
> +	struct page *page;
> +	int err;
> +
> +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> +		return vmf_error(-EINVAL);
> +
> +retry:
> +	page = find_lock_page(mapping, offset);
> +	if (!page) {
> +		page = secretmem_alloc_page(vmf->gfp_mask);
> +		if (!page)
> +			return VM_FAULT_OOM;
> +
> +		err = set_direct_map_invalid_noflush(page, 1);
> +		if (err)
> +			return vmf_error(err);

Haven't we leaked the page at this point?

> +		__SetPageUptodate(page);
> +		err = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);

At this point, doesn't the page contain data from the last person to use
the page?  ie we've leaked data to this process?  I don't see anywhere
that we write data to the page.