[PATCH 12/35] KVM: x86: Fix guest single-stepping while interruptible

Avi Kivity
2009-11-19T13:34:48Z

From: Jan Kiszka <jan.kiszka@web.de>

Commit 705c5323 opened the doors of hell by unconditionally injecting
single-step flags as long as guest_debug signaled this. This doesn't
work when the guest branches into some interrupt or exception handler
and triggers a vmexit with flag reloading.

Fix it by saving cs:rip when user space requests single-stepping and
restricting the trace flag injection to this guest code position.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 4 +++
arch/x86/kvm/x86.c | 47 +++++++++++++++++++++++---------------
2 files changed, 32 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 36f3b53..2536fbd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -371,6 +371,10 @@ struct kvm_vcpu_arch {
u64 mcg_status;
u64 mcg_ctl;
u64 *mce_banks;
+
+ /* used for guest single stepping over the given code position */
+ u16 singlestep_cs;
+ unsigned long singlestep_rip;
};

struct kvm_mem_alias {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bb842db..13f30aa 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -235,25 +235,6 @@ bool kvm_require_cpl(struct kvm_vcpu *vcpu, int required_cpl)
}
EXPORT_SYMBOL_GPL(kvm_require_cpl);

-unsigned long kvm_get_rflags(struct kvm_vcpu *vcpu)
-{
- unsigned long rflags;
-
- rflags = kvm_x86_ops->get_rflags(vcpu);
- if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
- rflags &= ~(unsigned long)(X86_EFLAGS_TF | X86_EFLAGS_RF);
- return rflags;
-}
-EXPORT_SYMBOL_GPL(kvm_get_rflags);
-
-void kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
-{
- if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
- rflags |= X86_EFLAGS_TF | X86_EFLAGS_RF;
- kvm_x86_ops->set_rflags(vcpu, rflags);
-}
-EXPORT_SYMBOL_GPL(kvm_set_rflags);
-
/*
* Load the pae pdptrs. Return true is they are all valid.
*/
@@ -4565,6 +4546,12 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
vcpu->arch.switch_db_regs = (vcpu->arch.dr7 & DR7_BP_EN_MASK);
}

+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+ vcpu->arch.singlestep_cs =
+ get_segment_selector(vcpu, VCPU_SREG_CS);
+ vcpu->arch.singlestep_rip = kvm_rip_read(vcpu);
+ }
+
/*
* Trigger an rflags update that will inject or remove the trace
* flags.
@@ -5031,6 +5018,28 @@ int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu)
return kvm_x86_ops->interrupt_allowed(vcpu);
}

+unsigned long kvm_get_rflags(struct kvm_vcpu *vcpu)
+{
+ unsigned long rflags;
+
+ rflags = kvm_x86_ops->get_rflags(vcpu);
+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
+ rflags &= ~(unsigned long)(X86_EFLAGS_TF | X86_EFLAGS_RF);
+ return rflags;
+}
+EXPORT_SYMBOL_GPL(kvm_get_rflags);
+
+void kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
+{
+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP &&
+ vcpu->arch.singlestep_cs ==
+ get_segment_selector(vcpu, VCPU_SREG_CS) &&
+ vcpu->arch.singlestep_rip == kvm_rip_read(vcpu))
+ rflags |= X86_EFLAGS_TF | X86_EFLAGS_RF;
+ kvm_x86_ops->set_rflags(vcpu, rflags);
+}
+EXPORT_SYMBOL_GPL(kvm_set_rflags);
+
EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_inj_virq);
EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_page_fault);
--
1.6.5.2