Re: [PATCH 1/3] KVM: arm64: Don't set PSTATE.SS when Software Step state is Active-pending

Reiji Watanabe
2022-09-14T06:13:48Z

Hi Marc,

Thank you for the review!

On Sat, Sep 10, 2022 at 3:36 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Fri, 09 Sep 2022 05:46:34 +0100,
> Reiji Watanabe <reijiw@google.com> wrote:
> >
> > Currently, PSTATE.SS is set on every guest entry if single-step is
> > enabled for the vCPU by userspace. However, it could cause extra
> > single-step execution without returning to userspace, which shouldn't
> > be performed, if the Software Step state at the last guest exit was
> > Active-pending (i.e. the last exit was not triggered by Software Step
> > exception, but by an asynchronous exception after the single-step
> > execution is performed).
> >
> > Fix this by not setting PSTATE.SS on guest entry if the Software
> > Step state at the last exit was Active-pending.
> >
> > Fixes: 337b99bf7edf ("KVM: arm64: guest debug, add support for single-step")
> > Signed-off-by: Reiji Watanabe <reijiw@google.com>
>
> Now that I'm a bit more clued about what the architecture actually
> mandates, I can try and review this patch.
>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 3 +++
> > arch/arm64/kvm/debug.c | 19 ++++++++++++++++++-
> > arch/arm64/kvm/guest.c | 1 +
> > arch/arm64/kvm/handle_exit.c | 2 ++
> > 4 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index e9c9388ccc02..4cf6eef02565 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -535,6 +535,9 @@ struct kvm_vcpu_arch {
> > #define IN_WFIT __vcpu_single_flag(sflags, BIT(3))
> > /* vcpu system registers loaded on physical CPU */
> > #define SYSREGS_ON_CPU __vcpu_single_flag(sflags, BIT(4))
> > +/* Software step state is Active-pending */
> > +#define DBG_SS_ACTIVE_PENDING __vcpu_single_flag(sflags, BIT(5))
> > +
> >
> > /* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
> > #define vcpu_sve_pffr(vcpu) (kern_hyp_va((vcpu)->arch.sve_state) + \
> > diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> > index 0b28d7db7c76..125cfb94b4ad 100644
> > --- a/arch/arm64/kvm/debug.c
> > +++ b/arch/arm64/kvm/debug.c
> > @@ -188,7 +188,16 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> > * debugging the system.
> > */
> > if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> > - *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
> > + /*
> > + * If the software step state at the last guest exit
> > + * was Active-pending, we don't set DBG_SPSR_SS so
> > + * that the state is maintained (to not run another
> > + * single-step until the pending Software Step
> > + * exception is taken).
> > + */
> > + if (!vcpu_get_flag(vcpu, DBG_SS_ACTIVE_PENDING))
> > + *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
> > +
> > mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
> > mdscr |= DBG_MDSCR_SS;
> > vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
> > @@ -279,6 +288,14 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
> > &vcpu->arch.debug_ptr->dbg_wcr[0],
> > &vcpu->arch.debug_ptr->dbg_wvr[0]);
> > }
> > +
> > + if ((vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) &&
> > + !(*vcpu_cpsr(vcpu) & DBG_SPSR_SS))
> > + /*
> > + * Mark the vcpu as ACTIVE_PENDING
> > + * until Software Step exception is confirmed.
>
> s/confirmed/taken/? This would match the comment in the previous hunk.

Yes, I will fix that.

>
> > + */
> > + vcpu_set_flag(vcpu, DBG_SS_ACTIVE_PENDING);
> > }
> > }
> >
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index f802a3b3f8db..2ff13a3f8479 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -937,6 +937,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
> > } else {
> > /* If not enabled clear all flags */
> > vcpu->guest_debug = 0;
> > + vcpu_clear_flag(vcpu, DBG_SS_ACTIVE_PENDING);
> > }
> >
> > out:
> > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > index bbe5b393d689..8e43b2668d67 100644
> > --- a/arch/arm64/kvm/handle_exit.c
> > +++ b/arch/arm64/kvm/handle_exit.c
> > @@ -154,6 +154,8 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
> >
> > if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
> > run->debug.arch.far = vcpu->arch.fault.far_el2;
> > + else if (ESR_ELx_EC(esr) == ESR_ELx_EC_SOFTSTP_LOW)
> > + vcpu_clear_flag(vcpu, DBG_SS_ACTIVE_PENDING);
>
> Can we write this as a switch/case statement?

Sure, I will change this to switch/case statement.

>
> >
> > return 0;
> > }
>
> I think we also need to do something if userspace decides to write to
> PSTATE as a result of a non-debug exit (such as a signal) when this
> DBG_SS_ACTIVE_PENDING is set. I came up with the following
> complicated, but not impossible scenario:
>
> - guest single step, PSTATE.SS=0
> - exit due to interrupt
> - DBG_SS_ACTIVE_PENDING set
> - reenter guest
> - exit again due to another interrupt
> - exit to userspace due to signal pending
> - userspace writes PSTATE.SS=1 for no good reason
> - we now have an inconsistent state between PSTATE.SS and the vcpu flags
>
> My gut feeling is that we need something like the vcpu flag being set
> to !PSTATE.SS if written while debug is enabled.
>
> Thoughts?

Ah, that's a good point.
Values that KVM is going to set in debug registers (e.g. MDSCR_EL1,
dbg_bcr, etc) at guest-entry cannot be changed by userspace via
SET_ONE_REG when debug is enabled. I'm inclined to apply the same
for PSTATE.SS (clear PSTATE.SS if the vcpu flag is set on guest entry,
and set PSTATE.SS to 1 otherwise). Since MDSCR_EL1 value that KVM is
going to set is not visible from userspace, changing Software-step
state when userspace updates PSTATE.SS might be a bit odd IMHO
(something odd anyway though).

Related to the above scenario, I found another bug (I think).
After guest exits with Active-not-pending (PSTATE.SS==1) due to an
interrupt, and then KVM exits to userspace due to signal pending,
if userspace disables single-step, PSTATE.SS will remain 1 on
subsequent guest entries (or it might have been originally 1, and
KVM might clear it. Most of the time it doesn't matter, and when the
guest is also using single-step, things will go wrong anyway though).

Considering those, I am thinking of changing the patch as follows,
- Change kvm_arm_setup_debug() to clear PSTATE.SS if the vcpu flag
(DBG_SS_ACTIVE_PENDING) is set, and set PSTATE.SS to 1 otherwise.
- Change save_guest_debug_regs()/restore_guest_debug_regs() to
save/restore the guest value of PSTATE.SS
(Add a new field in kvm_vcpu_arch.guest_debug_preserved to save
the guest value of PSTATE.SS)
keeping the other changes in the patch below.
- Clear DBG_SS_ACTIVE_PENDING in kvm_handle_guest_debug()
- Clear DBG_SS_ACTIVE_PENDING when userspace disables single-step

With this, PSTATE.SS value that KVM is going to set on guest-entry
won't be exposed to userspace, and PSTATE.SS value that is set by
userspace will not be used for the guest until single-step is
disabled (similar to MDSCR_EL1).

What do you think ?

Thank you,
Reiji