On 22 December 2015 at 09:55, Marc Zyngier <marc.zyngier@xxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> 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
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