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

[Tycho Andersen <tycho@docker.com>]

Hi Yisheng,

On Mon, Sep 11, 2017 at 03:24:09PM +0800, Yisheng Xie wrote:
> > +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

Yes, I think you're right. Oddly, the XPFO_READ_USER test works
correctly for me, but I think (?) should not because of this bug...

Tycho