linux-kernel - [PATCH v2 9/9] KVM: x86: never write to memory from kvm_vcpu_check

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Date:   Thu, 11 Aug 2022 17:06:05 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com, mlevitsk@...hat.com, vkuznets@...hat.com
Subject: [PATCH v2 9/9] KVM: x86: never write to memory from kvm_vcpu_check_block

kvm_vcpu_check_block() is called while not in TASK_RUNNING, and therefore
it cannot sleep.  Writing to guest memory is therefore forbidden, but it
can happen on AMD processors if kvm_check_nested_events() causes a vmexit.

Fortunately, all events that are caught by kvm_check_nested_events() are
also recognized by kvm_vcpu_has_events() through vendor callbacks such as
kvm_x86_interrupt_allowed() or kvm_x86_ops.nested_ops->has_events(), so
remove the call and postpone the actual processing to vcpu_block().

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/x86.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5e9358ea112b..9226fd536783 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10639,6 +10639,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 			return 1;
 	}
 
+	if (is_guest_mode(vcpu)) {
+		/*
+		 * Evaluate nested events before exiting the halted state.
+		 * This allows the halt state to be recorded properly in
+		 * the VMCS12's activity state field (AMD does not have
+		 * a similar field and a vmexit always causes a spurious
+		 * wakeup from HLT).
+		 */
+		kvm_check_nested_events(vcpu);
+	}
+
 	if (kvm_apic_accept_events(vcpu) < 0)
 		return 0;
 	switch(vcpu->arch.mp_state) {
@@ -10662,9 +10673,6 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 
 static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
 {
-	if (is_guest_mode(vcpu))
-		kvm_check_nested_events(vcpu);
-
 	return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
 		!vcpu->arch.apf.halted);
 }
-- 
2.31.1