Re: [Qemu-devel] [PATCH] target/s390x/kvm: Fix problem when running with SELinux under z/VM

[David Hildenbrand]

On 18.09.2017 09:43, Christian Borntraeger wrote:
> 
> 
> On 09/15/2017 04:36 PM, Thomas Huth wrote:
>> On 29.03.2017 16:25, Christian Borntraeger wrote:
>>> On 03/29/2017 04:21 PM, Thomas Huth wrote:
>>>> On 24.03.2017 10:39, Christian Borntraeger wrote:
>>>>> On 03/24/2017 10:26 AM, Thomas Huth wrote:
>>>>>> When running QEMU with KVM under z/VM, the memory for the guest
>>>>>> is allocated via legacy_s390_alloc() since the KVM_CAP_S390_COW
>>>>>> extension is not supported on z/VM. legacy_s390_alloc() then uses
>>>>>> mmap(... PROT_EXEC ...) for the guest memory - but this does not
>>>>>> work when running with SELinux enabled, mmap() fails and QEMU aborts
>>>>>> with the following error message:
>>>>>>
>>>>>>  cannot set up guest memory 's390.ram': Permission denied
>>>>>>
>>>>>> Looking at the other allocator function qemu_anon_ram_alloc(), it
>>>>>> seems like PROT_EXEC is normally not needed for allocating the
>>>>>> guest RAM, and indeed, the guest also starts successfully under
>>>>>> z/VM when we remove the PROT_EXEC from the legacy_s390_alloc()
>>>>>> function. So let's get rid of that flag here to be able to run
>>>>>> with SELinux under z/VM, too.
>>>>>
>>>>> Older z/VM versions do not provide the enhanced suppression on protection
>>>>> facility, which would result in guest failures as soon as the kernel
>>>>> starts dirty pages tracking by write protecting the pages via the page
>>>>> table. Some kernel release back (last time I checked) the PROT_EXEC was 
>>>>> necessary to prevent the dirty pages tracking from taking place. So this
>>>>> patch would break KVM in that case.
>>>>>
>>>>> Newer z/VMs (e.g. 6.3) do provide ESOP. SO the question is,
>>>>> why is KVM_CAP_S390_COW not set?
>>>>
>>>> I now had another look at this, and seems like the ESOP bit is indeed
>>>> not set in S390_lowcore.machine_flags here. According to /proc/sysinfo,
>>>> z/VM is version 6.1.0 here, so I guess that's just too old for ESOP?
>>>
>>> Yes, this was introduced with z/VM 6.3
>>
>> FWIW, the last version without ESOP, z/VM 6.2, is now end of life,
>> according to: http://www.vm.ibm.com/techinfo/lpmigr/vmleos.html
>> ... so I guess we could remove the legacy_s390_alloc() function now?
> 
> 
> I recently learned that you can buy some extended z/VM support not sure how
> long this will be available. In addition, ESOP was added with z10, so
> if we still care about z9 and older then this would break things on
> very very old boxes.

I wonder if that is really relevant anymore.

Existing user on such machines (I doubt there are many) can simply stick
to QEMU <= 2.10. Or do we actually expect people with such old
environments to use latest and grates QEMU versions?

We could add an error message an error out.

> 
> The pain/risk-to-break ratio seems to suggest to keep this "hack"
> for a while.


-- 

Thanks,

David