Re: [PATCH v4 3/3] virtio-mem: disallow mapping virtio-mem memory via /dev/mem

[David Hildenbrand <david@redhat.com>]

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 ...


> the virtio change itself is ok though:
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>

Thanks!


-- 
Thanks,

David / dhildenb

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