cxl/region: Fix a race condition in memory hotplug notifier

In the memory hotplug notifier function of the CXL region,
cxl_region_perf_attrs_callback(), the node ID is obtained by checking
the host address range of the region. However, the address range
information is not available when the region is registered in
devm_cxl_add_region(). Additionally, this information may be removed
or added under the protection of cxl_region_rwsem during runtime. If
the memory notifier is called for nodes other than that backed by the
region, a race condition may occur, potentially leading to a NULL
dereference or an invalid address range.

The race condition is addressed by checking the availability of the
address range information under the protection of cxl_region_rwsem. To
enhance code readability and use guard(), the relevant code has been
moved into a newly added function: cxl_region_nid().

Fixes: 067353a ("cxl/region: Add memory hotplug notifier for cxl region")
Signed-off-by: Huang, Ying <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Alison Schofield <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Bharata B Rao <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Ira Weiny <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Dave Jiang <[email protected]>

Author: yhuang-intel

drivers/cxl/core/region.c

@@ -2304,22 +2304,33 @@ static bool cxl_region_update_coordinates(struct cxl_region *cxlr, int nid)
 	return true;
 }
 
+static int cxl_region_nid(struct cxl_region *cxlr)
+{
+	struct cxl_region_params *p = &cxlr->params;
+	struct cxl_endpoint_decoder *cxled;
+	struct cxl_decoder *cxld;
+
+	guard(rwsem_read)(&cxl_region_rwsem);
+	cxled = p->targets[0];
+	if (!cxled)
+		return NUMA_NO_NODE;
+	cxld = &cxled->cxld;
+	return phys_to_target_node(cxld->hpa_range.start);
+}
+
 static int cxl_region_perf_attrs_callback(struct notifier_block *nb,
 					  unsigned long action, void *arg)
 {
 	struct cxl_region *cxlr = container_of(nb, struct cxl_region,
 					       memory_notifier);
-	struct cxl_region_params *p = &cxlr->params;
-	struct cxl_endpoint_decoder *cxled = p->targets[0];
-	struct cxl_decoder *cxld = &cxled->cxld;
 	struct memory_notify *mnb = arg;
 	int nid = mnb->status_change_nid;
 	int region_nid;
 
 	if (nid == NUMA_NO_NODE || action != MEM_ONLINE)
 		return NOTIFY_DONE;
 
-	region_nid = phys_to_target_node(cxld->hpa_range.start);
+	region_nid = cxl_region_nid(cxlr);
 	if (nid != region_nid)
 		return NOTIFY_DONE;