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

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Date:   Tue, 14 Sep 2021 13:57:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Wang <jasowang@...hat.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 3/3] virtio-mem: disallow mapping virtio-mem memory via
 /dev/mem

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@...el.com>
>>>> Signed-off-by: David Hildenbrand <david@...hat.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@...hat.com>
>>
>> Thanks!
>>
>>
>> -- 
>> Thanks,
>>
>> David / dhildenb
> 


-- 
Thanks,

David / dhildenb