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Peter Anvin" , Ingo Molnar , Thomas Gleixner , Paolo Bonzini , x86@kernel.org, Borislav Petkov , linux-kernel@vger.kernel.org, Maxim Levitsky Subject: [PATCH 2/3] KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present_queued Date: Wed, 13 Aug 2025 15:23:12 -0400 Message-ID: <20250813192313.132431-3-mlevitsk@redhat.com> In-Reply-To: <20250813192313.132431-1-mlevitsk@redhat.com> References: <20250813192313.132431-1-mlevitsk@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Content-Type: text/plain; charset="utf-8" Fix a semi theoretical race condition in reading of page_ready_pending in kvm_arch_async_page_present_queued. Only trust the value of page_ready_pending if the guest is about to enter guest mode (vcpu->mode). To achieve this, read the vcpu->mode using smp_load_acquire which is paired with smp_store_release in vcpu_enter_guest. Then only if vcpu_mode is IN_GUEST_MODE, trust the value of the page_ready_pending because it was written before and therefore its correct value is visible. Also if the above mentioned check is true, avoid raising the request on the target vCPU. Signed-off-by: Maxim Levitsky --- arch/x86/kvm/x86.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 9018d56b4b0a..3d45a4cd08a4 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -13459,9 +13459,14 @@ void kvm_arch_async_page_present(struct kvm_vcpu *= vcpu, =20 void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu) { - kvm_make_request(KVM_REQ_APF_READY, vcpu); - if (!vcpu->arch.apf.pageready_pending) + /* Pairs with smp_store_release in vcpu_enter_guest. */ + bool in_guest_mode =3D (smp_load_acquire(&vcpu->mode) =3D=3D IN_GUEST_MOD= E); + bool page_ready_pending =3D READ_ONCE(vcpu->arch.apf.pageready_pending); + + if (!in_guest_mode || !page_ready_pending) { + kvm_make_request(KVM_REQ_APF_READY, vcpu); kvm_vcpu_kick(vcpu); + } } =20 bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu) --=20 2.49.0