x86/xen: Probe target addresses in set_aliased_prot() before the hypercall

	pte_t pte;

	unsigned long pfn;

	struct page *page;

	unsigned char dummy;

	ptep = lookup_address((unsigned long)v, &level);

	BUG_ON(ptep == NULL);

	pte = pfn_pte(pfn, prot);

	/*

	 * Careful: update_va_mapping() will fail if the virtual address

	 * we're poking isn't populated in the page tables.  We don't

	 * need to worry about the direct map (that's always in the page

	 * tables), but we need to be careful about vmap space.  In

	 * particular, the top level page table can lazily propagate

	 * entries between processes, so if we've switched mms since we

	 * vmapped the target in the first place, we might not have the

	 * top-level page table entry populated.

	 *

	 * We disable preemption because we want the same mm active when

	 * we probe the target and when we issue the hypercall.  We'll

	 * have the same nominal mm, but if we're a kernel thread, lazy

	 * mm dropping could change our pgd.

	 *

	 * Out of an abundance of caution, this uses __get_user() to fault

	 * in the target address just in case there's some obscure case

	 * in which the target address isn't readable.

	 */

	preempt_disable();

	pagefault_disable();	/* Avoid warnings due to being atomic. */

	__get_user(dummy, (unsigned char __user __force *)v);

	pagefault_enable();

	if (HYPERVISOR_update_va_mapping((unsigned long)v, pte, 0))

		BUG();

				BUG();

	} else

		kmap_flush_unused();

	preempt_enable();

}

static void xen_alloc_ldt(struct desc_struct *ldt, unsigned entries)

	const unsigned entries_per_page = PAGE_SIZE / LDT_ENTRY_SIZE;

	int i;

	/*

	 * We need to mark the all aliases of the LDT pages RO.  We

	 * don't need to call vm_flush_aliases(), though, since that's

	 * only responsible for flushing aliases out the TLBs, not the

	 * page tables, and Xen will flush the TLB for us if needed.

	 *

	 * To avoid confusing future readers: none of this is necessary

	 * to load the LDT.  The hypervisor only checks this when the

	 * LDT is faulted in due to subsequent descriptor access.

	 */

	for(i = 0; i < entries; i += entries_per_page)

		set_aliased_prot(ldt + i, PAGE_KERNEL_RO);

}