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

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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>
This commit is contained in:
Ladi Prosek 2017-06-21 09:06:59 +02:00 committed by Paolo Bonzini
parent ab2f4d73eb
commit 9b61174793
1 changed files with 14 additions and 1 deletions
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15
arch/x86/kvm/svm.c
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@ -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),