linux-kernel - [PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

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Date:   Tue, 27 Jul 2021 13:06:20 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     stable@...r.kernel.org, Stas Sergeev <stsp2@...dex.ru>
Subject: [PATCH v2] KVM: x86: accept userspace interrupt only if no event is injected

Once an exception has been injected, any side effects related to
the exception (such as setting CR2 or DR6) have been taked place.
Therefore, once KVM sets the VM-entry interruption information
field or the AMD EVENTINJ field, the next VM-entry must deliver that
exception.

Pending interrupts are processed after injected exceptions, so
in theory it would not be a problem to use KVM_INTERRUPT when
an injected exception is present.  However, DOSEMU is using
run->ready_for_interrupt_injection to detect interrupt windows
and then using KVM_SET_SREGS/KVM_SET_REGS to inject the
interrupt manually.  For this to work, the interrupt window
must be delayed after the completion of the previous event
injection.

Cc: stable@...r.kernel.org
Reported-by: Stas Sergeev <stsp2@...dex.ru>
Tested-by: Stas Sergeev <stsp2@...dex.ru>
Fixes: 71cc849b7093 ("KVM: x86: Fix split-irqchip vs interrupt injection window request")
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
(cherry picked from commit 043264d97e9ab74cc9661c8b1f9c00c8ce24cad9)
---
 arch/x86/kvm/x86.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4116567f3d44..5e921f1e00db 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4358,8 +4358,18 @@ static int kvm_cpu_accept_dm_intr(struct kvm_vcpu *vcpu)

 static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
 {
+	/*
+	 * Do not cause an interrupt window exit if an exception
+	 * is pending or an event needs reinjection; userspace
+	 * might want to inject the interrupt manually using KVM_SET_REGS
+	 * or KVM_SET_SREGS.  For that to work, we must be at an
+	 * instruction boundary and with no events half-injected.
+	 */
 	return kvm_arch_interrupt_allowed(vcpu) &&
-		kvm_cpu_accept_dm_intr(vcpu);
+		kvm_cpu_accept_dm_intr(vcpu) &&
+	        !kvm_event_needs_reinjection(vcpu)
+	        !vcpu->arch.exception.pending;
+
 }

 static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
-- 
2.27.0