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

[Thomas Huth]

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?

 Thomas