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

Christian Borntraeger
2019-09-12T11:00:14Z

On 12.09.19 12:58, David Hildenbrand wrote:
> On 12.09.19 11:20, 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@redhat.com>
>>>> ---
>>>> 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).
>
> I would vote for doing this as well.

Yes, lets also do the designated initializer, add more text to the patch
description (or should we not?) add cc stable and I will pick a v2.
>
>>
>> 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.
>>
>
> That's indeed true.
>
> Reviewed-by: David Hildenbrand <david@redhat.com>
>
>> Thomas
>>
>
>