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authorDavid Hildenbrand <david@redhat.com>2021-11-08 18:35:50 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2021-11-09 10:02:52 -0800
commita9e7b8d4f663d13975e9bbe849735c8659c2a1f1 (patch)
tree32b90a2e2449ec2c8c0c6f42ea7db6ed2ddde81c /kernel
parentb78dfa059fdd7d0f791132dc0cef39ceafdfd62e (diff)
kernel/resource: disallow access to exclusive system RAM regions
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 with "IORESOURCE_SYSTEM_RAM | IORESOURCE_EXCLUSIVE", even if not busy and even if "iomem=relaxed" is set. Introduce EXCLUSIVE_SYSTEM_RAM to make it easier for applicable drivers to depend on this setting in their Kconfig. 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 add_memory_driver_managed(). Link: https://lkml.kernel.org/r/20210920142856.17758-3-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Dan Williams <dan.j.williams@intel.com> Cc: Andy Shevchenko <andy.shevchenko@gmail.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Hanjun Guo <guohanjun@huawei.com> Cc: Jason Wang <jasowang@redhat.com> Cc: "Michael S. Tsirkin" <mst@redhat.com> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/resource.c29
1 files 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;
#endif
/*
- * 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 |
+ IORESOURCE_EXCLUSIVE;
bool skip_children = false, err = false;
int size = PAGE_SIZE;
struct resource *p;
- if (!strict_iomem_checks)
- return false;
-
addr = addr & PAGE_MASK;
read_lock(&resource_lock);
for_each_resource(&iomem_resource, p, skip_children) {
- /*
- * We can probably skip the resources without
- * IORESOURCE_IO attribute?
- */
if (p->start >= addr + size)
break;
if (p->end < addr) {
@@ -1748,11 +1745,23 @@ 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))
continue;
if (IS_ENABLED(CONFIG_IO_STRICT_DEVMEM)
|| p->flags & IORESOURCE_EXCLUSIVE) {