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
