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 12:52:52 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Thomas Huth <thuth@...hat.com>,
        David Hildenbrand <david@...hat.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 12.09.19 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@...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).

Yes doing something like

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index c19a24e940a1..b1f6f434af5d 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4332,7 +4332,7 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
        }
        case KVM_S390_INTERRUPT: {
                struct kvm_s390_interrupt s390int;
-               struct kvm_s390_irq s390irq;
+               struct kvm_s390_irq s390irq = {};
 
                if (copy_from_user(&s390int, argp, sizeof(s390int)))
                        return -EFAULT;

would certainly be ok as well, but

> 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.

as long as we we this interface we should fix it and we should do the
pfault thing correctly. 
Maybe we should start to deprecate this interface and remove it.
For the time being Thomas fix is certainly good enough. We might want to
add the designated initializer as an additional safety barrier.