Re: [PATCH 16/17] KVM: PPC: MPIC: Add support for KVM_IRQ_LINE

Alexander Graf
2013-04-25T14:49:23Z

On 25.04.2013, at 13:30, Alexander Graf wrote:

>
> On 19.04.2013, at 20:51, Scott Wood wrote:
>
>> On 04/19/2013 09:06:27 AM, Alexander Graf wrote:
>>> Now that all pieces are in place for reusing generic irq infrastructure,
>>> we can copy x86's implementation of KVM_IRQ_LINE irq injection and simply
>>> reuse it for PPC, as it will work there just as well.
>>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>> ---
>>> arch/powerpc/include/uapi/asm/kvm.h | 1 +
>>> arch/powerpc/kvm/powerpc.c | 13 +++++++++++++
>>> 2 files changed, 14 insertions(+), 0 deletions(-)
>>> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
>>> index 3537bf3..dbb2ac2 100644
>>> --- a/arch/powerpc/include/uapi/asm/kvm.h
>>> +++ b/arch/powerpc/include/uapi/asm/kvm.h
>>> @@ -26,6 +26,7 @@
>>> #define __KVM_HAVE_SPAPR_TCE
>>> #define __KVM_HAVE_PPC_SMT
>>> #define __KVM_HAVE_IRQCHIP
>>> +#define __KVM_HAVE_IRQ_LINE
>>> struct kvm_regs {
>>> __u64 pc;
>>> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
>>> index c431fea..874c106 100644
>>> --- a/arch/powerpc/kvm/powerpc.c
>>> +++ b/arch/powerpc/kvm/powerpc.c
>>> @@ -33,6 +33,7 @@
>>> #include <asm/cputhreads.h>
>>> #include <asm/irqflags.h>
>>> #include "timing.h"
>>> +#include "irq.h"
>>> #include "../mm/mmu_decl.h"
>>> #define CREATE_TRACE_POINTS
>>> @@ -945,6 +946,18 @@ static int kvm_vm_ioctl_get_pvinfo(struct kvm_ppc_pvinfo *pvinfo)
>>> return 0;
>>> }
>>> +int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_event,
>>> + bool line_status)
>>> +{
>>> + if (!irqchip_in_kernel(kvm))
>>> + return -ENXIO;
>>> +
>>> + irq_event->status = kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID,
>>> + irq_event->irq, irq_event->level,
>>> + line_status);
>>> + return 0;
>>> +}
>>
>> As Paul noted in the XICS patchset, this could reference an MPIC that has gone away if the user never attached any vcpus and then closed the MPIC fd. It's not a reasonable use case, but it could be used malicously to get the kernel to access a bad pointer. The irqchip_in_kernel check helps somewhat, but it's meant for ensuring that the creation has happened -- it's racy if used for ensuring that destruction hasn't happened.
>>
>> The problem is rooted in the awkwardness of performing an operation that logically should be on the MPIC fd, but is instead being done on the vm fd.
>>
>> I think these three steps would fix it (the first two seem like things we should be doing anyway):
>> - During MPIC destruction, make sure MPIC deregisters all routes that reference it.
>> - In kvm_set_irq(), do not release the RCU read lock until after the set() function has been called.
>> - Do not hook up kvm_send_userspace_msi() to MPIC or other new irqchips, as that bypasses the RCU lock. It could be supported as a device fd ioctl if desired, or it could be reworked to operate on an RCU-managed list of MSI handlers, though MPIC really doesn't need this at all.
>
> Can't we just add an RCU lock in the send_userspace_msi case? I don't think we should handle MSIs any differently from normal IRQs.

In fact I'm having a hard time verifying that we're always accessing things with proper locks held. I'm pretty sure we're missing a few cases.

So how about we delay mpic destruction to vm destruction? We simply add one user too many when we spawn the mpic and put it on vm_destruct. That way users _can_ destroy mpics, but they will only be really free'd once the vm is also gone.

Alex