From: Boris Ostrovsky boris.ostrovsky@oracle.com

There is a potential race in record_steal_time() between setting host-local vcpu->arch.st.steal.preempted to zero (i.e. clearing KVM_VCPU_PREEMPTED) and propagating this value to the guest with kvm_write_guest_cached(). Between those two events the guest may still see KVM_VCPU_PREEMPTED in its copy of kvm_steal_time, set KVM_VCPU_FLUSH_TLB and assume that hypervisor will do the right thing. Which it won't.

Instad of copying, we should map kvm_steal_time and that will guarantee atomicity of accesses to @preempted.

This is part of CVE-2019-3016.

Signed-off-by: Boris Ostrovsky boris.ostrovsky@oracle.com Reviewed-by: Joao Martins joao.m.martins@oracle.com Cc: stable@vger.kernel.org Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- arch/x86/kvm/x86.c | 51 ++++++++++++++++++++++++++++++--------------------- 1 file changed, 30 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 0795bc8..f1845df 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2581,45 +2581,47 @@ static void kvm_vcpu_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa)

static void record_steal_time(struct kvm_vcpu *vcpu) { + struct kvm_host_map map; + struct kvm_steal_time *st; + if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;

- if (unlikely(kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.st.stime, - &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)))) + /* -EAGAIN is returned in atomic context so we can just return. */ + if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, + &map, &vcpu->arch.st.cache, false)) return;

+ st = map.hva + + offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + /* * Doing a TLB flush here, on the guest's behalf, can avoid * expensive IPIs. */ trace_kvm_pv_tlb_flush(vcpu->vcpu_id, - vcpu->arch.st.steal.preempted & KVM_VCPU_FLUSH_TLB); - if (xchg(&vcpu->arch.st.steal.preempted, 0) & KVM_VCPU_FLUSH_TLB) + st->preempted & KVM_VCPU_FLUSH_TLB); + if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB) kvm_vcpu_flush_tlb(vcpu, false);

- if (vcpu->arch.st.steal.version & 1) - vcpu->arch.st.steal.version += 1; /* first time write, random junk */ + vcpu->arch.st.steal.preempted = 0;

- vcpu->arch.st.steal.version += 1; + if (st->version & 1) + st->version += 1; /* first time write, random junk */

- kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime, - &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)); + st->version += 1;

smp_wmb();

- vcpu->arch.st.steal.steal += current->sched_info.run_delay - + st->steal += current->sched_info.run_delay - vcpu->arch.st.last_steal; vcpu->arch.st.last_steal = current->sched_info.run_delay;

- kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime, - &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)); - smp_wmb();

- vcpu->arch.st.steal.version += 1; + st->version += 1;

- kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime, - &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)); + kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, false); }

int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) @@ -3501,18 +3503,25 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) { + struct kvm_host_map map; + struct kvm_steal_time *st; + if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;

if (vcpu->arch.st.steal.preempted) return;

- vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED; + if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, &map, + &vcpu->arch.st.cache, true)) + return; + + st = map.hva + + offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + + st->preempted = vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED;

- kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime, - &vcpu->arch.st.steal.preempted, - offsetof(struct kvm_steal_time, preempted), - sizeof(vcpu->arch.st.steal.preempted)); + kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, true); }

void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)