[PATCH 1/2] arm: KVM: Do not update PC if the trap handler has updated it

[Peter Maydell]

On 22 December 2015 at 09:55, Marc Zyngier <marc.zyngier at arm.com> wrote:
> Assuming we trap a coprocessor access, and decide that the access
> is illegal, we will inject an exception in the guest. In this
> case, we shouldn't increment the PC, or the vcpu will miss the
> first instruction of the handler, leading to a mildly confused
> guest.
>
> Solve this by snapshoting PC before the access is performed,
> and checking if it has moved or not before incrementing it.
>
> Reported-by: Shannon Zhao <shannon.zhao at linaro.org>
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> ---
>  arch/arm/kvm/coproc.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/kvm/coproc.c b/arch/arm/kvm/coproc.c
> index f3d88dc..f4ad2f2 100644
> --- a/arch/arm/kvm/coproc.c
> +++ b/arch/arm/kvm/coproc.c
> @@ -447,12 +447,22 @@ static int emulate_cp15(struct kvm_vcpu *vcpu,
>                 r = find_reg(params, cp15_regs, ARRAY_SIZE(cp15_regs));
>
>         if (likely(r)) {
> +               unsigned long pc = *vcpu_pc(vcpu);
> +
>                 /* If we don't have an accessor, we should never get here! */
>                 BUG_ON(!r->access);
>
>                 if (likely(r->access(vcpu, params, r))) {
> -                       /* Skip instruction, since it was emulated */
> -                       kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> +                       /*
> +                        * Skip the instruction if it was emulated
> +                        * without PC having changed. This allows us
> +                        * to detect a fault being injected
> +                        * (incrementing the PC here would cause the
> +                        * vcpu to skip the first instruction of its
> +                        * fault handler).
> +                        */
> +                       if (pc == *vcpu_pc(vcpu))
> +                               kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));

Won't this result in our incorrectly skipping the first insn
in the fault handler if the original offending instruction
was itself the first insn in the fault handler?

thanks
-- PMM