Re: [PATCH v3 3/3] virtio-mem: disallow mapping virtio-mem memory via /dev/mem

["Williams, Dan J" <dan.j.williams@intel.com>]

On Tue, 2021-08-31 at 22:21 +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.
> 
> 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

Thanks for this. It keeps the special case out of the implementation
while not compromising integrity for VIRTIO_MEM.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

>         help
>          This driver provides access to virtio-mem paravirtualized memory
>          devices, allowing to hotplug and hotunplug memory.
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index b91bc810a87e..c2d93492cf0f 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -2523,8 +2523,10 @@ static int virtio_mem_create_resource(struct virtio_mem *vm)
>         if (!name)
>                 return -ENOMEM;
>  
> +       /* Disallow mapping device memory via /dev/mem completely. */
>         vm->parent_resource = __request_mem_region(vm->addr, vm->region_size,
> -                                                  name, IORESOURCE_SYSTEM_RAM);
> +                                                  name, IORESOURCE_SYSTEM_RAM |
> +                                                  IORESOURCE_EXCLUSIVE);
>         if (!vm->parent_resource) {
>                 kfree(name);
>                 dev_warn(&vm->vdev->dev, "could not reserve device region\n");