Commit message: KVM: Fix out of range accesses to memslots

Reset the LRU slot if it becomes invalid when deleting a memslot to fix
an out-of-bounds/use-after-free access when searching through memslots.

Explicitly check for there being no used slots in search_memslots(), and
in the caller of s390's approximation variant.

Fixes: 36947254e5f9 ("KVM: Dynamically size memslot array based on number of used slots")
Reported-by: Qian Cai <cai@lca.pw>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320205546.2396-2-sean.j.christopherson@intel.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
CVE: CVE-2020-36313
CWE: CWE-416
Vulnerability description: An issue was discovered in the Linux kernel before 5.7. The KVM subsystem allows out-of-range access to memslots after a deletion, aka CID-0774a964ef56. This affects arch/s390/kvm/kvm-s390.c, include/linux/kvm_host.h, and virt/kvm/kvm_main.c.

Project: linux
File: kvm-s390.c

static int kvm_s390_get_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,
			     u8 *res, unsigned long bufsize)
{
	unsigned long mem_end, cur_gfn, next_gfn, hva, pgstev;
	struct kvm_memslots *slots = kvm_memslots(kvm);
	struct kvm_memory_slot *ms;

	cur_gfn = kvm_s390_next_dirty_cmma(slots, args->start_gfn);
	ms = gfn_to_memslot(kvm, cur_gfn);
	args->count = 0;
	args->start_gfn = cur_gfn;
	if (!ms)
		return 0;
	next_gfn = kvm_s390_next_dirty_cmma(slots, cur_gfn + 1);
	mem_end = slots->memslots[0].base_gfn + slots->memslots[0].npages;

	while (args->count < bufsize) {
		hva = gfn_to_hva(kvm, cur_gfn);
		if (kvm_is_error_hva(hva))
			return 0;
		/* Decrement only if we actually flipped the bit to 0 */
		if (test_and_clear_bit(cur_gfn - ms->base_gfn, kvm_second_dirty_bitmap(ms)))
			atomic64_dec(&kvm->arch.cmma_dirty_pages);
		if (get_pgste(kvm->mm, hva, &pgstev) < 0)
			pgstev = 0;
		/* Save the value */
		res[args->count++] = (pgstev >> 24) & 0x43;
		/* If the next bit is too far away, stop. */
		if (next_gfn > cur_gfn + KVM_S390_MAX_BIT_DISTANCE)
			return 0;
		/* If we reached the previous "next", find the next one */
		if (cur_gfn == next_gfn)
			next_gfn = kvm_s390_next_dirty_cmma(slots, cur_gfn + 1);
		/* Reached the end of memory or of the buffer, stop */
		if ((next_gfn >= mem_end) ||
		    (next_gfn - args->start_gfn >= bufsize))
			return 0;
		cur_gfn++;
		/* Reached the end of the current memslot, take the next one. */
		if (cur_gfn - ms->base_gfn >= ms->npages) {
			ms = gfn_to_memslot(kvm, cur_gfn);
			if (!ms)
				return 0;
		}
	}
	return 0;
}

Project: linux
File: kvm_main.c

static inline void kvm_memslot_delete(struct kvm_memslots *slots,
				      struct kvm_memory_slot *memslot)
{
	struct kvm_memory_slot *mslots = slots->memslots;
	int i;

	if (WARN_ON(slots->id_to_index[memslot->id] == -1))
		return;

	slots->used_slots--;

	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
		mslots[i] = mslots[i + 1];
		slots->id_to_index[mslots[i].id] = i;
	}
	mslots[i] = *memslot;
	slots->id_to_index[memslot->id] = -1;
}

Project: linux
File: kvm_host.h

search_memslots(struct kvm_memslots *slots, gfn_t gfn)
{
	int start = 0, end = slots->used_slots;
	int slot = atomic_read(&slots->lru_slot);
	struct kvm_memory_slot *memslots = slots->memslots;

	if (gfn >= memslots[slot].base_gfn &&
	    gfn < memslots[slot].base_gfn + memslots[slot].npages)
		return &memslots[slot];

	while (start < end) {
		slot = start + (end - start) / 2;

		if (gfn >= memslots[slot].base_gfn)
			end = slot;
		else
			start = slot + 1;
	}

	if (gfn >= memslots[start].base_gfn &&
	    gfn < memslots[start].base_gfn + memslots[start].npages) {
		atomic_set(&slots->lru_slot, start);
		return &memslots[start];
	}

	return NULL;
}