linux-kernel - [PATCH v4 10/12] KVM: x86: never write to memory from kvm_vcpu_check

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Message-ID: <20220921003201.1441511-11-seanjc@google.com>
Date:   Wed, 21 Sep 2022 00:31:59 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH v4 10/12] KVM: x86: never write to memory from kvm_vcpu_check_block()

From: Paolo Bonzini <pbonzini@...hat.com>

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().

Opportunistically honor the return of kvm_check_nested_events().  KVM
punted on the check in kvm_vcpu_running() because the only error path is
if vmx_complete_nested_posted_interrupt() fails, in which case KVM exits
to userspace with "internal error" i.e. the VM is likely dead anyways so
it wasn't worth overloading the return of kvm_vcpu_running().

Add the check mostly so that KVM is consistent with itself; the return of
the call via kvm_apic_accept_events()=>kvm_check_nested_events() that
immediately follows  _is_ checked.

Reported-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
[sean: check and handle return of kvm_check_nested_events()]
Signed-off-by: Sean Christopherson <seanjc@...gle.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 dcc675d4e44b..8aeacbc2bff9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10815,6 +10815,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 			return 1;
 	}
 
+	/*
+	 * 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 VM-Exit always
+	 * causes a spurious wakeup from HLT).
+	 */
+	if (is_guest_mode(vcpu)) {
+		if (kvm_check_nested_events(vcpu) < 0)
+			return 0;
+	}
+
 	if (kvm_apic_accept_events(vcpu) < 0)
 		return 0;
 	switch(vcpu->arch.mp_state) {
@@ -10837,9 +10848,6 @@ 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);
 }
-- 
2.37.3.968.ga6b4b080e4-goog