linux-kernel - Re: [PATCH] KVM: s390: Do not leak kernel stack data in the KVM_S390

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Date:   Thu, 12 Sep 2019 11:20:24 +0200
From:   Thomas Huth <thuth@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     Cornelia Huck <cohuck@...hat.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: s390: Do not leak kernel stack data in the
 KVM_S390_INTERRUPT ioctl

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@...hat.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).

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.

 Thomas