linux-kernel - [PATCH 5.15 056/262] KVM: s390: Ensure kvm_arch_no

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Message-Id: <20220307091704.094220587@linuxfoundation.org>
Date:   Mon,  7 Mar 2022 10:16:40 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 056/262] KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU

From: Sean Christopherson <seanjc@...gle.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@...ibm.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Message-Id: <20211009021236.4122790-4-seanjc@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.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