Commit message: KVM: Validate userspace_addr of memslot when registered

This way, we can avoid checking the user space address many times when
we read the guest memory.

Although we can do the same for write if we check which slots are
writable, we do not care write now: reading the guest memory happens
more often than writing.

[avi: change VERIFY_READ to VERIFY_WRITE]

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>
CVE: CVE-2013-1943
CWE: CWE-20
Vulnerability description: The KVM subsystem in the Linux kernel before 3.0 does not check whether kernel addresses are specified during allocation of memory slots for use in a guest's physical address space, which allows local users to gain privileges or obtain sensitive information from kernel memory via a crafted application, related to arch/x86/kvm/paging_tmpl.h and virt/kvm/kvm_main.c.

Project: linux
File: kvm_main.c

int __kvm_set_memory_region(struct kvm *kvm,
			    struct kvm_userspace_memory_region *mem,
			    int user_alloc)
{
	int r;
	gfn_t base_gfn;
	unsigned long npages;
	unsigned long i;
	struct kvm_memory_slot *memslot;
	struct kvm_memory_slot old, new;
	struct kvm_memslots *slots, *old_memslots;

	r = -EINVAL;
	/* General sanity checks */
	if (mem->memory_size & (PAGE_SIZE - 1))
 		goto out;
 	if (mem->guest_phys_addr & (PAGE_SIZE - 1))
 		goto out;
	/* We can read the guest memory with __xxx_user() later on. */
	if (user_alloc &&
	    ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
	     !access_ok(VERIFY_WRITE, mem->userspace_addr, mem->memory_size)))
 		goto out;
 	if (mem->slot >= KVM_MEMORY_SLOTS + KVM_PRIVATE_MEM_SLOTS)
 		goto out;
	if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
		goto out;

	memslot = &kvm->memslots->memslots[mem->slot];
	base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
	npages = mem->memory_size >> PAGE_SHIFT;

	r = -EINVAL;
	if (npages > KVM_MEM_MAX_NR_PAGES)
		goto out;

	if (!npages)
		mem->flags &= ~KVM_MEM_LOG_DIRTY_PAGES;

	new = old = *memslot;

	new.id = mem->slot;
	new.base_gfn = base_gfn;
	new.npages = npages;
	new.flags = mem->flags;

	/* Disallow changing a memory slot's size. */
	r = -EINVAL;
	if (npages && old.npages && npages != old.npages)
		goto out_free;

	/* Check for overlaps */
	r = -EEXIST;
	for (i = 0; i < KVM_MEMORY_SLOTS; ++i) {
		struct kvm_memory_slot *s = &kvm->memslots->memslots[i];

		if (s == memslot || !s->npages)
			continue;
		if (!((base_gfn + npages <= s->base_gfn) ||
		      (base_gfn >= s->base_gfn + s->npages)))
			goto out_free;
	}

	/* Free page dirty bitmap if unneeded */
	if (!(new.flags & KVM_MEM_LOG_DIRTY_PAGES))
		new.dirty_bitmap = NULL;

	r = -ENOMEM;

	/* Allocate if a slot is being created */
#ifndef CONFIG_S390
	if (npages && !new.rmap) {
		new.rmap = vzalloc(npages * sizeof(*new.rmap));

		if (!new.rmap)
			goto out_free;

		new.user_alloc = user_alloc;
		new.userspace_addr = mem->userspace_addr;
	}
	if (!npages)
		goto skip_lpage;

	for (i = 0; i < KVM_NR_PAGE_SIZES - 1; ++i) {
		unsigned long ugfn;
		unsigned long j;
		int lpages;
		int level = i + 2;

		/* Avoid unused variable warning if no large pages */
		(void)level;

		if (new.lpage_info[i])
			continue;

		lpages = 1 + ((base_gfn + npages - 1)
			     >> KVM_HPAGE_GFN_SHIFT(level));
		lpages -= base_gfn >> KVM_HPAGE_GFN_SHIFT(level);

		new.lpage_info[i] = vzalloc(lpages * sizeof(*new.lpage_info[i]));

		if (!new.lpage_info[i])
			goto out_free;

		if (base_gfn & (KVM_PAGES_PER_HPAGE(level) - 1))
			new.lpage_info[i][0].write_count = 1;
		if ((base_gfn+npages) & (KVM_PAGES_PER_HPAGE(level) - 1))
			new.lpage_info[i][lpages - 1].write_count = 1;
		ugfn = new.userspace_addr >> PAGE_SHIFT;
		/*
		 * If the gfn and userspace address are not aligned wrt each
		 * other, or if explicitly asked to, disable large page
		 * support for this slot
		 */
		if ((base_gfn ^ ugfn) & (KVM_PAGES_PER_HPAGE(level) - 1) ||
		    !largepages_enabled)
			for (j = 0; j < lpages; ++j)
				new.lpage_info[i][j].write_count = 1;
	}

skip_lpage:

	/* Allocate page dirty bitmap if needed */
	if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
		if (kvm_create_dirty_bitmap(&new) < 0)
			goto out_free;
		/* destroy any largepage mappings for dirty tracking */
	}
#else  /* not defined CONFIG_S390 */
	new.user_alloc = user_alloc;
	if (user_alloc)
		new.userspace_addr = mem->userspace_addr;
#endif /* not defined CONFIG_S390 */

	if (!npages) {
		r = -ENOMEM;
		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
		if (!slots)
			goto out_free;
		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
		if (mem->slot >= slots->nmemslots)
			slots->nmemslots = mem->slot + 1;
		slots->generation++;
		slots->memslots[mem->slot].flags |= KVM_MEMSLOT_INVALID;

		old_memslots = kvm->memslots;
		rcu_assign_pointer(kvm->memslots, slots);
		synchronize_srcu_expedited(&kvm->srcu);
		/* From this point no new shadow pages pointing to a deleted
		 * memslot will be created.
		 *
		 * validation of sp->gfn happens in:
		 * 	- gfn_to_hva (kvm_read_guest, gfn_to_pfn)
		 * 	- kvm_is_visible_gfn (mmu_check_roots)
		 */
		kvm_arch_flush_shadow(kvm);
		kfree(old_memslots);
	}

	r = kvm_arch_prepare_memory_region(kvm, &new, old, mem, user_alloc);
	if (r)
		goto out_free;

	/* map the pages in iommu page table */
	if (npages) {
		r = kvm_iommu_map_pages(kvm, &new);
		if (r)
			goto out_free;
	}

	r = -ENOMEM;
	slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
	if (!slots)
		goto out_free;
	memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
	if (mem->slot >= slots->nmemslots)
		slots->nmemslots = mem->slot + 1;
	slots->generation++;

	/* actual memory is freed via old in kvm_free_physmem_slot below */
	if (!npages) {
		new.rmap = NULL;
		new.dirty_bitmap = NULL;
		for (i = 0; i < KVM_NR_PAGE_SIZES - 1; ++i)
			new.lpage_info[i] = NULL;
	}

	slots->memslots[mem->slot] = new;
	old_memslots = kvm->memslots;
	rcu_assign_pointer(kvm->memslots, slots);
	synchronize_srcu_expedited(&kvm->srcu);

	kvm_arch_commit_memory_region(kvm, mem, old, user_alloc);

	kvm_free_physmem_slot(&old, &new);
	kfree(old_memslots);

	return 0;

out_free:
	kvm_free_physmem_slot(&new, &old);
out:
	return r;

}