[PATCH v4 2/3] kernel/resource: disallow access to exclusive system RAM regions

From: David Hildenbrand
Date: Thu Sep 02 2021 - 12:09:45 EST

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.

In contrast to physical devices, like a DIMM, the virtio-mem driver
is required to actually make use of any of the device-provided memory,
because it performs the handshake with the hypervisor. virtio-mem memory
cannot simply be access via /dev/mem without a driver.

There is no safe way to:
a) Access plugged memory blocks via /dev/mem, as they might contain
unplugged holes or might get silently unplugged by the virtio-mem
driver and consequently turned inaccessible.
b) Access unplugged memory blocks via /dev/mem because the virtio-mem
driver is required to make them actually accessible first.

The virtio-spec states that unplugged memory blocks MUST NOT be
written, and only selected unplugged memory blocks MAY be read. We want
to make sure, this is the case in sane environments -- where the
virtio-mem driver was loaded.

We want to make sure that in a sane environment, nobody "accidentially"
accesses unplugged memory inside the device managed region. For example,
a user might spot a memory region in /proc/iomem and try accessing it via
/dev/mem via gdb or dumping it via something else. By the time the mmap()
happens, the memory might already have been removed by the virtio-mem
driver silently: the mmap() would succeeed and user space might
accidentially access unplugged memory.

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.

In an ideal world, we would mark the whole region as busy ("owned by a
driver") and exclude it; however, that would be wrong, as we don't
really have actual system RAM at these ranges added to Linux ("busy system
RAM"). Instead, we want to mark such ranges as "not actual busy system RAM
but still soft-reserved and prepared by a driver for future use."

Let's teach iomem_is_exclusive() to reject access to any range
and even if "iomem=relaxed" is set.

For now, there are no applicable ranges and we'll modify virtio-mem next to
properly set IORESOURCE_EXCLUSIVE on the parent resource container it
creates to contain all actual busy system RAM added via

Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
kernel/resource.c | 29 +++++++++++++++++++----------
1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/kernel/resource.c b/kernel/resource.c
index 2999f57da38c..5ad3eba619ba 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1719,26 +1719,23 @@ static int strict_iomem_checks;

- * check if an address is reserved in the iomem resource tree
- * returns true if reserved, false if not reserved.
+ * Check if an address is exclusive to the kernel and must not be mapped to
+ * user space, for example, via /dev/mem.
+ *
+ * Returns true if exclusive to the kernel, otherwise returns false.
bool iomem_is_exclusive(u64 addr)
+ const unsigned int exclusive_system_ram = IORESOURCE_SYSTEM_RAM |
bool skip_children = false, err = false;
int size = PAGE_SIZE;
struct resource *p;

- if (!strict_iomem_checks)
- return false;
addr = addr & PAGE_MASK;

for_each_resource(&iomem_resource, p, skip_children) {
- /*
- * We can probably skip the resources without
- * IORESOURCE_IO attribute?
- */
if (p->start >= addr + size)
if (p->end < addr) {
@@ -1747,12 +1744,24 @@ bool iomem_is_exclusive(u64 addr)
skip_children = false;

+ /*
+ * IORESOURCE_SYSTEM_RAM resources are exclusive if
+ * IORESOURCE_EXCLUSIVE is set, even if they
+ * are not busy and even if "iomem=relaxed" is set. The
+ * responsible driver dynamically adds/removes system RAM within
+ * such an area and uncontrolled access is dangerous.
+ */
+ if ((p->flags & exclusive_system_ram) == exclusive_system_ram) {
+ err = true;
+ break;
+ }
* A resource is exclusive if IORESOURCE_EXCLUSIVE is set
* or CONFIG_IO_STRICT_DEVMEM is enabled and the
* resource is busy.
- if ((p->flags & IORESOURCE_BUSY) == 0)
+ if (!strict_iomem_checks || !(p->flags & IORESOURCE_BUSY))