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Subject[PATCH 5.15 056/262] KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU
Date
From: Sean Christopherson <seanjc@google.com>

[ Upstream commit 6f390916c4fb359507d9ac4bf1b28a4f8abee5c0 ]

Wrap s390's halt_poll_max_steal with READ_ONCE and snapshot the result of
kvm_arch_no_poll() in kvm_vcpu_block() to avoid a mostly-theoretical,
largely benign bug on s390 where the result of kvm_arch_no_poll() could
change due to userspace modifying halt_poll_max_steal while the vCPU is
blocking. The bug is largely benign as it will either cause KVM to skip
updating halt-polling times (no_poll toggles false=>true) or to update
halt-polling times with a slightly flawed block_ns.

Note, READ_ONCE is unnecessary in the current code, add it in case the
arch hook is ever inlined, and to provide a hint that userspace can
change the param at will.

Fixes: 8b905d28ee17 ("KVM: s390: provide kvm_arch_no_poll function")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/s390/kvm/kvm-s390.c | 2 +-
virt/kvm/kvm_main.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 9a8c086528f56..402597f9d0505 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3447,7 +3447,7 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
{
/* do not poll with more than halt_poll_max_steal percent of steal time */
if (S390_lowcore.avg_steal_timer * 100 / (TICK_USEC << 12) >=
- halt_poll_max_steal) {
+ READ_ONCE(halt_poll_max_steal)) {
vcpu->stat.halt_no_poll_steal++;
return true;
}
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13aff136e6eef..f8b42e19bc775 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3222,6 +3222,7 @@ update_halt_poll_stats(struct kvm_vcpu *vcpu, u64 poll_ns, bool waited)
*/
void kvm_vcpu_block(struct kvm_vcpu *vcpu)
{
+ bool halt_poll_allowed = !kvm_arch_no_poll(vcpu);
ktime_t start, cur, poll_end;
bool waited = false;
u64 block_ns;
@@ -3229,7 +3230,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
kvm_arch_vcpu_blocking(vcpu);

start = cur = poll_end = ktime_get();
- if (vcpu->halt_poll_ns && !kvm_arch_no_poll(vcpu)) {
+ if (vcpu->halt_poll_ns && halt_poll_allowed) {
ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);

++vcpu->stat.generic.halt_attempted_poll;
@@ -3284,7 +3285,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
update_halt_poll_stats(
vcpu, ktime_to_ns(ktime_sub(poll_end, start)), waited);

- if (!kvm_arch_no_poll(vcpu)) {
+ if (halt_poll_allowed) {
if (!vcpu_valid_wakeup(vcpu)) {
shrink_halt_poll_ns(vcpu);
} else if (vcpu->kvm->max_halt_poll_ns) {
--
2.34.1


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