Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

Yisheng Xie
2017-09-11T07:24:35Z

Hi Tycho,

On 2017/9/8 1:36, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@canonical.com>
>
> This patch adds support for XPFO which protects against 'ret2dir' kernel
> attacks. The basic idea is to enforce exclusive ownership of page frames
> by either the kernel or userspace, unless explicitly requested by the
> kernel. Whenever a page destined for userspace is allocated, it is
> unmapped from physmap (the kernel's page table). When such a page is
> reclaimed from userspace, it is mapped back to physmap.
>
> Additional fields in the page_ext struct are used for XPFO housekeeping,
> specifically:
> - two flags to distinguish user vs. kernel pages and to tag unmapped
> pages.
> - a reference counter to balance kmap/kunmap operations.
> - a lock to serialize access to the XPFO fields.
>
> This patch is based on the work of Vasileios P. Kemerlis et al. who
> published their work in this paper:
> http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf
>
> [...]
> +void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp)
> +{
> + int i, flush_tlb = 0;
> + struct xpfo *xpfo;
> +
> + if (!static_branch_unlikely(&xpfo_inited))
> + return;
> +
> + for (i = 0; i < (1 << order); i++) {
> + xpfo = lookup_xpfo(page + i);
> + if (!xpfo)
> + continue;
> +
> + WARN(test_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags),
> + "xpfo: unmapped page being allocated\n");
> +
> + /* Initialize the map lock and map counter */
> + if (unlikely(!xpfo->inited)) {
> + spin_lock_init(&xpfo->maplock);
> + atomic_set(&xpfo->mapcount, 0);
> + xpfo->inited = true;
> + }
> + WARN(atomic_read(&xpfo->mapcount),
> + "xpfo: already mapped page being allocated\n");
> +
> + if ((gfp & GFP_HIGHUSER) == GFP_HIGHUSER) {
> + /*
> + * Tag the page as a user page and flush the TLB if it
> + * was previously allocated to the kernel.
> + */
> + if (!test_and_set_bit(XPFO_PAGE_USER, &xpfo->flags))
> + flush_tlb = 1;

I'm not sure whether I am miss anything, however, when the page was previously allocated
to kernel, should we unmap the physmap (the kernel's page table) here? For we allocate
the page to user now

Yisheng Xie
Thanks