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SubjectRe: [PATCH v4 3/3] virtio-mem: disallow mapping virtio-mem memory via /dev/mem
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On 14.09.21 13:52, Michael S. Tsirkin wrote:
> On Tue, Sep 14, 2021 at 11:45:25AM +0200, David Hildenbrand wrote:
>> On 03.09.21 09:02, Michael S. Tsirkin wrote:
>>> On Thu, Sep 02, 2021 at 06:09:19PM +0200, David Hildenbrand wrote:
>>>> We don't want user space to be able to map virtio-mem device memory
>>>> directly (e.g., via /dev/mem) in order to have guarantees that in a sane
>>>> setup we'll never accidentially access unplugged memory within the
>>>> device-managed region of a virtio-mem device, just as required by the
>>>> virtio-spec.
>>>>
>>>> As soon as the virtio-mem driver is loaded, the device region is visible
>>>> in /proc/iomem via the parent device region. From that point on user space
>>>> is aware of the device region and we want to disallow mapping anything
>>>> inside that region (where we will dynamically (un)plug memory) until
>>>> the driver has been unloaded cleanly and e.g., another driver might take
>>>> over.
>>>>
>>>> By creating our parent IORESOURCE_SYSTEM_RAM resource with
>>>> IORESOURCE_EXCLUSIVE, we will disallow any /dev/mem access to our
>>>> device region until the driver was unloaded cleanly and removed the
>>>> parent region. This will work even though only some memory blocks are
>>>> actually currently added to Linux and appear as busy in the resource tree.
>>>>
>>>> So access to the region from user space is only possible
>>>> a) if we don't load the virtio-mem driver.
>>>> b) after unloading the virtio-mem driver cleanly.
>>>>
>>>> Don't build virtio-mem if access to /dev/mem cannot be restricticted --
>>>> if we have CONFIG_DEVMEM=y but CONFIG_STRICT_DEVMEM is not set.
>>>>
>>>> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>
>>>
>>>> ---
>>>> drivers/virtio/Kconfig | 1 +
>>>> drivers/virtio/virtio_mem.c | 4 +++-
>>>> 2 files changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
>>>> index ce1b3f6ec325..ff80cd03f1d1 100644
>>>> --- a/drivers/virtio/Kconfig
>>>> +++ b/drivers/virtio/Kconfig
>>>> @@ -101,6 +101,7 @@ config VIRTIO_MEM
>>>> depends on MEMORY_HOTPLUG_SPARSE
>>>> depends on MEMORY_HOTREMOVE
>>>> depends on CONTIG_ALLOC
>>>> + depends on !DEVMEM || STRICT_DEVMEM
>>>> help
>>>> This driver provides access to virtio-mem paravirtualized memory
>>>> devices, allowing to hotplug and hotunplug memory.
>>>
>>> It would be nicer if there was a symbol in the MEMORY_ namespace
>>> we can depend on exported by mm and depending on !DEVMEM ||
>>> STRICT_DEVMEM.
>>>
>>> E.g.
>>>
>>> config MEMORY_EXCLUSIVE
>>> def_bool y
>>> depends on !DEVMEM || STRICT_DEVMEM
>>>
>>> and then in virtio
>>> depends on MEMORY_EXCLUSIVE
>>>
>>>
>>
>> Yes, but I'm not able to come up with an expressive name. MEMORY_EXCLUSIVE
>> can be highly misleading ...
>
>
> I mean ... it enables IORESOURCE_EXCLUSIVE ... but ok ...
> MEMORY_EXCLUSIVE_KERNEL_MAP ?

It enables IORESOURCE_EXCLUSIVE for IORESOURCE_SYSTEM_RAM ... and
notably not for IORESOURCE_MEM.

MEMORY_EXCLUSIVE_SYSTEM_RAM ?

>
>>
>>> the virtio change itself is ok though:
>>>
>>> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>>
>> Thanks!
>>
>>
>> --
>> Thanks,
>>
>> David / dhildenb
>


--
Thanks,

David / dhildenb

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