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

[Michael S. Tsirkin]

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@xxxxxxxxx>
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > 
> > 
> > > ---
> > >   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 ?

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