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Date:   Thu, 16 May 2019 15:02:00 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Alexander Graf <graf@...zon.com>,
        Filippo Sironi <sironi@...zon.de>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <borntraeger@...ibm.com>, <boris.ostrovsky@...cle.com>,
        <cohuck@...hat.com>, <konrad.wilk@...cle.com>,
        <xen-devel@...ts.xenproject.org>, <vasu.srinivasan@...cle.com>
Subject: Re: [Xen-devel] [PATCH v2 1/2] KVM: Start populating /sys/hypervisor
 with KVM entries

On 16/05/2019 14:50, Alexander Graf wrote:
> On 14.05.19 08:16, Filippo Sironi wrote:
>> Start populating /sys/hypervisor with KVM entries when we're running on
>> KVM. This is to replicate functionality that's available when we're
>> running on Xen.
>>
>> Start with /sys/hypervisor/uuid, which users prefer over
>> /sys/devices/virtual/dmi/id/product_uuid as a way to recognize a virtual
>> machine, since it's also available when running on Xen HVM and on Xen PV
>> and, on top of that doesn't require root privileges by default.
>> Let's create arch-specific hooks so that different architectures can
>> provide different implementations.
>>
>> Signed-off-by: Filippo Sironi <sironi@...zon.de>
> I think this needs something akin to
>
>   https://www.kernel.org/doc/Documentation/ABI/stable/sysfs-hypervisor-xen
>
> to document which files are available.
>
>> ---
>> v2:
>> * move the retrieval of the VM UUID out of uuid_show and into
>>   kvm_para_get_uuid, which is a weak function that can be overwritten
>>
>>  drivers/Kconfig              |  2 ++
>>  drivers/Makefile             |  2 ++
>>  drivers/kvm/Kconfig          | 14 ++++++++++++++
>>  drivers/kvm/Makefile         |  1 +
>>  drivers/kvm/sys-hypervisor.c | 30 ++++++++++++++++++++++++++++++
>>  5 files changed, 49 insertions(+)
>>  create mode 100644 drivers/kvm/Kconfig
>>  create mode 100644 drivers/kvm/Makefile
>>  create mode 100644 drivers/kvm/sys-hypervisor.c
>>
> [...]
>
>> +
>> +__weak const char *kvm_para_get_uuid(void)
>> +{
>> +	return NULL;
>> +}
>> +
>> +static ssize_t uuid_show(struct kobject *obj,
>> +			 struct kobj_attribute *attr,
>> +			 char *buf)
>> +{
>> +	const char *uuid = kvm_para_get_uuid();
>> +	return sprintf(buf, "%s\n", uuid);
> The usual return value for the Xen /sys/hypervisor interface is
> "<denied>".

This string comes straight from Xen.

It was an effort to reduce the quantity of interesting fingerprintable
data accessable by default to unprivileged guests.

See
https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=a2fc8d514df2b38c310d4f4432fe06520b0769ed

~Andrew

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