Re: [PATCH] KVM: s390: Do not leak kernel stack data in the KVM_S390_INTERRUPT ioctl

[Janosch Frank]

On 9/12/19 11:20 AM, Thomas Huth wrote:
> On 12/09/2019 11.14, David Hildenbrand wrote:
>> On 12.09.19 11:00, Thomas Huth wrote:
>>> When the userspace program runs the KVM_S390_INTERRUPT ioctl to inject
>>> an interrupt, we convert them from the legacy struct kvm_s390_interrupt
>>> to the new struct kvm_s390_irq via the s390int_to_s390irq() function.
>>> However, this function does not take care of all types of interrupts
>>> that we can inject into the guest later (see do_inject_vcpu()). Since we
>>> do not clear out the s390irq values before calling s390int_to_s390irq(),
>>> there is a chance that we copy unwanted data from the kernel stack
>>> into the guest memory later if the interrupt data has not been properly
>>> initialized by s390int_to_s390irq().
>>>
>>> Specifically, the problem exists with the KVM_S390_INT_PFAULT_INIT
>>> interrupt: s390int_to_s390irq() does not handle it, but the function
>>> __deliver_pfault_init() will later copy the uninitialized stack data
>>> from the ext.ext_params2 into the guest memory.
>>>
>>> Fix it by handling that interrupt type in s390int_to_s390irq(), too.
>>> And while we're at it, make sure that s390int_to_s390irq() now
>>> directly returns -EINVAL for unknown interrupt types, so that we
>>> do not run into this problem again in case we add more interrupt
>>> types to do_inject_vcpu() sometime in the future.
>>>
>>> Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
>>> ---
>>>  arch/s390/kvm/interrupt.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>>
>>> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
>>> index 3e7efdd9228a..165dea4c7f19 100644
>>> --- a/arch/s390/kvm/interrupt.c
>>> +++ b/arch/s390/kvm/interrupt.c
>>> @@ -1960,6 +1960,16 @@ int s390int_to_s390irq(struct kvm_s390_interrupt *s390int,
>>>  	case KVM_S390_MCHK:
>>>  		irq->u.mchk.mcic = s390int->parm64;
>>>  		break;
>>> +	case KVM_S390_INT_PFAULT_INIT:
>>> +		irq->u.ext.ext_params = s390int->parm;
>>> +		irq->u.ext.ext_params2 = s390int->parm64;
>>> +		break;
>>> +	case KVM_S390_RESTART:
>>> +	case KVM_S390_INT_CLOCK_COMP:
>>> +	case KVM_S390_INT_CPU_TIMER:
>>> +		break;
>>> +	default:
>>> +		return -EINVAL;
>>>  	}
>>>  	return 0;
>>>  }
>>>
>>
>> Wouldn't a safe fix be to initialize the struct to zero in the caller?
> 
> That's of course possible, too. But that means that we always have to
> zero out the whole structure, so that's a little bit more of overhead
> (well, it likely doesn't matter for such a legacy ioctl).

Or memset it in s390int_to_s390irq so the caller doesn't need to know.
That should be fast enough for such a small struct.

> 
> But the more important question: Do we then still care of fixing the
> PFAULT_INIT interrupt here? Since it requires a parameter, the "case
> KVM_S390_INT_PFAULT_INIT:" part would be required here anyway.

Let's wait for Christian's opinion on that.

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