From: David Hildenbrand <david@xxxxxxxxxx> Subject: virtio-mem: disallow mapping virtio-mem memory via /dev/mem 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. Link: https://lkml.kernel.org/r/20210920142856.17758-4-david@xxxxxxxxxx Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Hanjun Guo <guohanjun@xxxxxxxxxx> Cc: Jason Wang <jasowang@xxxxxxxxxx> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- drivers/virtio/Kconfig | 1 + drivers/virtio/virtio_mem.c | 4 +++- 2 files changed, 4 insertions(+), 1 deletion(-) --- a/drivers/virtio/Kconfig~virtio-mem-disallow-mapping-virtio-mem-memory-via-dev-mem +++ a/drivers/virtio/Kconfig @@ -101,6 +101,7 @@ config VIRTIO_MEM depends on MEMORY_HOTPLUG depends on MEMORY_HOTREMOVE depends on CONTIG_ALLOC + depends on EXCLUSIVE_SYSTEM_RAM help This driver provides access to virtio-mem paravirtualized memory devices, allowing to hotplug and hotunplug memory. --- a/drivers/virtio/virtio_mem.c~virtio-mem-disallow-mapping-virtio-mem-memory-via-dev-mem +++ a/drivers/virtio/virtio_mem.c @@ -2672,8 +2672,10 @@ static int virtio_mem_create_resource(st 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"); _
