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

Christian Borntraeger
2019-09-12T10:47:22Z

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@redhat.com>

Cc: stable

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