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 12:47:13 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Thomas Huth <thuth@...hat.com>,
        Janosch Frank <frankja@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        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 11:00, 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().

You mean by using the migration callbacks to get all interrupts back to 
userspace?



> 
> 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.


Shouldnt we add some more detailed description how this can happen?
Something like
"By using the KVM_S390_INTERRUPT ioctl with a KVM_S390_INT_PFAULT_INIT
interrupt followed by the KVM_S390_GET_IRQ_STATE ioctl the user can
extract a value from the kernel stack."


> 
> 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>

Cc: stable

Reviewed-by: Christian Borntraeger <borntraeger@...ibm.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;
>  }
>