Whenever we get an -EFAULT, we failed to read in guest 2 physical
address space. Such addressing exceptions are reported via a program
intercept to the nested hypervisor.
We faked the intercept, we have to return to guest 2. Instead, right
now we would be returning -EFAULT from the intercept handler, eventually
crashing the VM.
Addressing exceptions can only happen if the g2->g3 page tables
reference invalid g2 addresses (say, either a table or the final page is
not accessible - so something that basically never happens in sane
environments.
Identified by manual code inspection.
Fixes: a3508fbe9dc6 ("KVM: s390: vsie: initial support for nested virtualization")
Cc: <[email protected]> # v4.8+
Signed-off-by: David Hildenbrand <[email protected]>
---
arch/s390/kvm/vsie.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index 076090f9e666..4f6c22d72072 100644
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -1202,6 +1202,7 @@ static int vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
scb_s->iprcc = PGM_ADDRESSING;
scb_s->pgmilc = 4;
scb_s->gpsw.addr = __rewind_psw(scb_s->gpsw, 4);
+ rc = 1;
}
return rc;
}
--
2.25.1
On Fri, 3 Apr 2020 17:30:47 +0200
David Hildenbrand <[email protected]> wrote:
> Whenever we get an -EFAULT, we failed to read in guest 2 physical
> address space. Such addressing exceptions are reported via a program
> intercept to the nested hypervisor.
>
> We faked the intercept, we have to return to guest 2. Instead, right
> now we would be returning -EFAULT from the intercept handler,
> eventually crashing the VM.
>
> Addressing exceptions can only happen if the g2->g3 page tables
> reference invalid g2 addresses (say, either a table or the final page
> is not accessible - so something that basically never happens in sane
> environments.
>
> Identified by manual code inspection.
>
> Fixes: a3508fbe9dc6 ("KVM: s390: vsie: initial support for nested
> virtualization") Cc: <[email protected]> # v4.8+
> Signed-off-by: David Hildenbrand <[email protected]>
> ---
> arch/s390/kvm/vsie.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 076090f9e666..4f6c22d72072 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -1202,6 +1202,7 @@ static int vsie_run(struct kvm_vcpu *vcpu,
> struct vsie_page *vsie_page) scb_s->iprcc = PGM_ADDRESSING;
> scb_s->pgmilc = 4;
> scb_s->gpsw.addr = __rewind_psw(scb_s->gpsw, 4);
> + rc = 1;
> }
> return rc;
> }
so, the reason why we never noticed this issue before is simply that
nobody tried running a misbehaving nested guest?
Reviewed-by: Claudio Imbrenda <[email protected]>
On 07.04.20 13:00, Claudio Imbrenda wrote:
> On Fri, 3 Apr 2020 17:30:47 +0200
> David Hildenbrand <[email protected]> wrote:
>
>> Whenever we get an -EFAULT, we failed to read in guest 2 physical
>> address space. Such addressing exceptions are reported via a program
>> intercept to the nested hypervisor.
>>
>> We faked the intercept, we have to return to guest 2. Instead, right
>> now we would be returning -EFAULT from the intercept handler,
>> eventually crashing the VM.
>>
>> Addressing exceptions can only happen if the g2->g3 page tables
>> reference invalid g2 addresses (say, either a table or the final page
>> is not accessible - so something that basically never happens in sane
>> environments.
>>
>> Identified by manual code inspection.
>>
>> Fixes: a3508fbe9dc6 ("KVM: s390: vsie: initial support for nested
>> virtualization") Cc: <[email protected]> # v4.8+
>> Signed-off-by: David Hildenbrand <[email protected]>
>> ---
>> arch/s390/kvm/vsie.c | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>> index 076090f9e666..4f6c22d72072 100644
>> --- a/arch/s390/kvm/vsie.c
>> +++ b/arch/s390/kvm/vsie.c
>> @@ -1202,6 +1202,7 @@ static int vsie_run(struct kvm_vcpu *vcpu,
>> struct vsie_page *vsie_page) scb_s->iprcc = PGM_ADDRESSING;
>> scb_s->pgmilc = 4;
>> scb_s->gpsw.addr = __rewind_psw(scb_s->gpsw, 4);
>> + rc = 1;
>> }
>> return rc;
>> }
>
> so, the reason why we never noticed this issue before is simply that
> nobody tried running a misbehaving nested guest?
Yes, actually, a misbehaving nested hypervisor.
>
> Reviewed-by: Claudio Imbrenda <[email protected]>
>
Thanks!
--
Thanks,
David / dhildenb