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x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed

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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>
This commit is contained in:
Boris Ostrovsky 2019-12-05 03:45:32 +00:00 committed by Paolo Bonzini
parent 917248144d
commit b043138246
1 changed files with 30 additions and 21 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

51
arch/x86/kvm/x86.c
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@ -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;
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));
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_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, true);
}
void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)