Treat EMULTYPE_SKIP failures on SEV guests as unhandleable emulation
instead of simply resuming the guest, and drop the hack-a-fix which
effects that behavior for the INT3/INTO injection path. If KVM can't
skip an instruction for which KVM has already done partial emulation,
resuming the guest is undesirable as doing so may corrupt guest state.
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/svm.c | 12 +-----------
arch/x86/kvm/x86.c | 9 +++++++--
2 files changed, 8 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 39ce680013c4..fc2cd5585349 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -364,8 +364,6 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask)
svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK;
}
-static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
- void *insn, int insn_len);
static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
bool commit_side_effects)
@@ -386,14 +384,6 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
}
if (!svm->next_rip) {
- /*
- * FIXME: Drop this when kvm_emulate_instruction() does the
- * right thing and treats "can't emulate" as outright failure
- * for EMULTYPE_SKIP.
- */
- if (svm_check_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0) != X86EMUL_CONTINUE)
- return 0;
-
if (unlikely(!commit_side_effects))
old_rflags = svm->vmcb->save.rflags;
@@ -4752,7 +4742,7 @@ static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
*/
if (unlikely(!insn)) {
if (emul_type & EMULTYPE_SKIP)
- return X86EMUL_RETRY_INSTR;
+ return X86EMUL_UNHANDLEABLE;
kvm_queue_exception(vcpu, UD_VECTOR);
return X86EMUL_PROPAGATE_FAULT;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f897d582d560..1f4a8fbc5390 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8858,8 +8858,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
bool writeback = true;
r = kvm_check_emulate_insn(vcpu, emulation_type, insn, insn_len);
- if (r != X86EMUL_CONTINUE)
- return 1;
+ if (r != X86EMUL_CONTINUE) {
+ if (r == X86EMUL_RETRY_INSTR || r == X86EMUL_PROPAGATE_FAULT)
+ return 1;
+
+ WARN_ON_ONCE(r != X86EMUL_UNHANDLEABLE);
+ return handle_emulation_failure(vcpu, emulation_type);
+ }
vcpu->arch.l1tf_flush_l1d = true;
--
2.42.0.rc2.253.gd59a3bf2b4-googOn 8/24/23 20:36, Sean Christopherson wrote:
> Treat EMULTYPE_SKIP failures on SEV guests as unhandleable emulation
> instead of simply resuming the guest, and drop the hack-a-fix which
> effects that behavior for the INT3/INTO injection path. If KVM can't
> skip an instruction for which KVM has already done partial emulation,
> resuming the guest is undesirable as doing so may corrupt guest state.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/svm/svm.c | 12 +-----------
> arch/x86/kvm/x86.c | 9 +++++++--
> 2 files changed, 8 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 39ce680013c4..fc2cd5585349 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -364,8 +364,6 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask)
> svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK;
>
> }
> -static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
> - void *insn, int insn_len);
>
> static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
> bool commit_side_effects)
> @@ -386,14 +384,6 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
> }
>
> if (!svm->next_rip) {
> - /*
> - * FIXME: Drop this when kvm_emulate_instruction() does the
> - * right thing and treats "can't emulate" as outright failure
> - * for EMULTYPE_SKIP.
> - */
> - if (svm_check_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0) != X86EMUL_CONTINUE)
> - return 0;
> -
> if (unlikely(!commit_side_effects))
> old_rflags = svm->vmcb->save.rflags;
>
> @@ -4752,7 +4742,7 @@ static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
> */
> if (unlikely(!insn)) {
> if (emul_type & EMULTYPE_SKIP)
> - return X86EMUL_RETRY_INSTR;
> + return X86EMUL_UNHANDLEABLE;
Trying to follow this, bear with me...
This results in an "emulation failure" which fills out all the KVM
userspace exit information in prepare_emulation_failure_exit(). But
because of the return 0 in handle_emulation_failure(), in the end this
ends up just acting like the first patch because we exit out
svm_update_soft_interrupt_rip() early and the instruction just gets retried?
Thanks,
Tom
>
> kvm_queue_exception(vcpu, UD_VECTOR);
> return X86EMUL_PROPAGATE_FAULT;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f897d582d560..1f4a8fbc5390 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8858,8 +8858,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> bool writeback = true;
>
> r = kvm_check_emulate_insn(vcpu, emulation_type, insn, insn_len);
> - if (r != X86EMUL_CONTINUE)
> - return 1;
> + if (r != X86EMUL_CONTINUE) {
> + if (r == X86EMUL_RETRY_INSTR || r == X86EMUL_PROPAGATE_FAULT)
> + return 1;
> +
> + WARN_ON_ONCE(r != X86EMUL_UNHANDLEABLE);
> + return handle_emulation_failure(vcpu, emulation_type);
> + }
>
> vcpu->arch.l1tf_flush_l1d = true;
>
On Fri, Aug 25, 2023, Tom Lendacky wrote:
> On 8/24/23 20:36, Sean Christopherson wrote:
> > Treat EMULTYPE_SKIP failures on SEV guests as unhandleable emulation
> > instead of simply resuming the guest, and drop the hack-a-fix which
> > effects that behavior for the INT3/INTO injection path. If KVM can't
> > skip an instruction for which KVM has already done partial emulation,
> > resuming the guest is undesirable as doing so may corrupt guest state.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > arch/x86/kvm/svm/svm.c | 12 +-----------
> > arch/x86/kvm/x86.c | 9 +++++++--
> > 2 files changed, 8 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 39ce680013c4..fc2cd5585349 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -364,8 +364,6 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask)
> > svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK;
> > }
> > -static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
> > - void *insn, int insn_len);
> > static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
> > bool commit_side_effects)
> > @@ -386,14 +384,6 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
> > }
> > if (!svm->next_rip) {
> > - /*
> > - * FIXME: Drop this when kvm_emulate_instruction() does the
> > - * right thing and treats "can't emulate" as outright failure
> > - * for EMULTYPE_SKIP.
> > - */
> > - if (svm_check_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0) != X86EMUL_CONTINUE)
> > - return 0;
> > -
> > if (unlikely(!commit_side_effects))
> > old_rflags = svm->vmcb->save.rflags;
> > @@ -4752,7 +4742,7 @@ static int svm_check_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
> > */
> > if (unlikely(!insn)) {
> > if (emul_type & EMULTYPE_SKIP)
> > - return X86EMUL_RETRY_INSTR;
> > + return X86EMUL_UNHANDLEABLE;
>
> Trying to follow this, bear with me...
>
> This results in an "emulation failure" which fills out all the KVM userspace
> exit information in prepare_emulation_failure_exit(). But because of the
> return 0 in handle_emulation_failure(), in the end this ends up just acting
> like the first patch because we exit out svm_update_soft_interrupt_rip()
> early and the instruction just gets retried?
Yep. It's a bit more labyrinthian than I'd like, but the soft injection already
relies on this behavior to handle the case where x86_decode_emulated_instruction()
fails. That's a much more theoretical path, but I'm pretty sure if could trigger
if the guest is replacing the INT3 from a different vCPU and KVM's emulator doesn't
know how to decode the new instruction.
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