From: Boris Ostrovsky boris.ostrovsky@oracle.com
commit b043138246a41064527cf019a3d51d9f015e9796 upstream.
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 Signed-off-by: Paolo Bonzini pbonzini@redhat.com [bwh: Backported to 4.19: No tracepoint in record_steal_time().] Signed-off-by: Ben Hutchings ben.hutchings@codethink.co.uk Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/x86.c | 49 +++++++++++++++++++++++++++------------------- 1 file changed, 29 insertions(+), 20 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 6916f46909ab0..d77822e03ff6b 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2397,43 +2397,45 @@ 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. */ - if (xchg(&vcpu->arch.st.steal.preempted, 0) & 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) @@ -3272,18 +3274,25 @@ 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)