Commit 9b611747 authored Jun 21, 2017 by Ladi Prosek Committed by Paolo Bonzini Jun 27, 2017

KVM: SVM: hide TF/RF flags used by NMI singlestep



These flags are used internally by SVM so it's cleaner to not leak
them to callers of svm_get_rflags. This is similar to how the TF
flag is handled on KVM_GUESTDBG_SINGLESTEP by kvm_get_rflags and
kvm_set_rflags.

Without this change, the flags may propagate from host VMCB to nested
VMCB or vice versa while singlestepping over a nested VM enter/exit,
and then get stuck in inappropriate places.

Example: NMI singlestepping is enabled while running L1 guest. The
instruction to step over is VMRUN and nested vmrun emulation stashes
rflags to hsave->save.rflags. Then if singlestepping is disabled
while still in L2, TF/RF will be cleared from the nested VMCB but the
next nested VM exit will restore them from hsave->save.rflags and
cause an unexpected DB exception.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent ab2f4d73

arch/x86/kvm/svm.c

+14 −1

Original line number	Diff line number	Diff line
		@@ -1725,11 +1725,24 @@ static void svm_vcpu_unblocking(struct kvm_vcpu *vcpu)

		static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
		{
		return to_svm(vcpu)->vmcb->save.rflags;
		struct vcpu_svm *svm = to_svm(vcpu);
		unsigned long rflags = svm->vmcb->save.rflags;

		if (svm->nmi_singlestep) {
		/* Hide our flags if they were not set by the guest */
		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF))
		rflags &= ~X86_EFLAGS_TF;
		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_RF))
		rflags &= ~X86_EFLAGS_RF;
		}
		return rflags;
		}

		static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
		{
		if (to_svm(vcpu)->nmi_singlestep)
		rflags \|= (X86_EFLAGS_TF \| X86_EFLAGS_RF);

		/*
		* Any change of EFLAGS.VM is accompanied by a reload of SS
		* (caused by either a task switch or an inter-privilege IRET),

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