Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Mike,

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:
> For instance, the following example will create an uncached mapping (error
> handling is omitted):
> 
> 	fd = memfd_secret(SECRETMEM_UNCACHED);
> 	ftruncate(fd, MAP_SIZE);
> 	ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
[...]
> +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 ret = 0;
> +
> +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> +		return vmf_error(-EINVAL);
> +
> +	page = find_get_entry(mapping, offset);
> +	if (!page) {
> +		page = secretmem_alloc_page(vmf->gfp_mask);
> +		if (!page)
> +			return vmf_error(-ENOMEM);
> +
> +		ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
> +		if (unlikely(ret))
> +			goto err_put_page;
> +
> +		ret = set_direct_map_invalid_noflush(page);
> +		if (ret)
> +			goto err_del_page_cache;
> +
> +		addr = (unsigned long)page_address(page);
> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> +
> +		__SetPageUptodate(page);
> +
> +		ret = VM_FAULT_LOCKED;
> +	}
> +
> +	vmf->page = page;
> +	return ret;
> +
> +err_del_page_cache:
> +	delete_from_page_cache(page);
> +err_put_page:
> +	put_page(page);
> +	return vmf_error(ret);
> +}
> +
> +static const struct vm_operations_struct secretmem_vm_ops = {
> +	.fault = secretmem_fault,
> +};
> +
> +static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct secretmem_ctx *ctx = file->private_data;
> +	unsigned long mode = ctx->mode;
> +	unsigned long len = vma->vm_end - vma->vm_start;
> +
> +	if (!mode)
> +		return -EINVAL;
> +
> +	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
> +		return -EINVAL;
> +
> +	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
> +		return -EAGAIN;
> +
> +	switch (mode) {
> +	case SECRETMEM_UNCACHED:
> +		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +		fallthrough;
> +	case SECRETMEM_EXCLUSIVE:
> +		vma->vm_ops = &secretmem_vm_ops;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	vma->vm_flags |= VM_LOCKED;
> +
> +	return 0;
> +}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

-- 
Catalin

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