On Mon, Aug 16, 2021 at 7:25 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
virtio-mem dynamically exposes memory inside a device memory region as
system RAM to Linux, coordinating with the hypervisor which parts are
actually "plugged" and consequently usable/accessible. On the one hand, the
virtio-mem driver adds/removes whole memory blocks, creating/removing busy
IORESOURCE_SYSTEM_RAM resources, on the other hand, it logically (un)plugs
memory inside added memory blocks, dynamically either exposing them to
the buddy or hiding them from the buddy and marking them PG_offline.
virtio-mem wants to make sure that in a sane environment, nobody
"accidentially" accesses unplugged memory inside the device managed
region. After /proc/kcore has been sanitized and /dev/kmem has been
removed, /dev/mem is the remaining interface that still allows uncontrolled
access to the device-managed region of virtio-mem devices from user
There is no known sane use case for mapping virtio-mem device memory
via /dev/mem while virtio-mem driver concurrently (un)plugs memory inside
that region. So once the driver was loaded and detected the device
along the device-managed region, we just want to disallow any access via
/dev/mem to it.
Let's add the basic infrastructure to exclude some physical memory
regions completely from /dev/mem access, on any architecture and under
any system configuration (independent of CONFIG_STRICT_DEVMEM and
independent of "iomem=").
I'm certainly on team "/dev/mem considered harmful", but this approach
feels awkward. It feels wrong for being non-committal about whether
CONFIG_STRICT_DEVMEM is in wide enough use that the safety can be
turned on all the time, and the configuration option dropped, or there
are users clinging onto /dev/mem where they expect to be able to build
a debug kernel to turn all of these restrictions off, even the
virtio-mem ones. This splits the difference and says some /dev/mem
accesses are always disallowed for "reasons", but I could say the same
thing about pmem, there's no sane reason to allow /dev/mem which has
no idea about the responsibilities of properly touching pmem to get
access to it.