[PATCH] cxl/region: Guard against a missing peer mapping

[Alison Schofield <alison.schofield@intel.com>]

When two endpoints share a dport, both must be mapped by the port.

sashiko-bot reports that a missing peer mapping could result in a NULL
pointer dereference during region creation.

No caller can reach this today. Guard it anyway so a future violation
of the mapping requirement fails region creation rather than causing
an oops.

Reported-by: sashiko-bot@kernel.org
Link: https://sashiko.dev/#/patchset/cover.1787255388.git.alison.schofield%40intel.com?part=3
Assisted-by: Claude:claude-opus-5
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---
 drivers/cxl/core/region.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 27e63e6dab7c..75b8092e6dc2 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -1367,7 +1367,8 @@ static int check_last_peer(struct cxl_endpoint_decoder *cxled,
 	/*
 	 * If this position wants to share a dport with the last endpoint mapped
 	 * then that endpoint, at index 'position - distance', must also be
-	 * mapped by this dport.
+	 * mapped by this dport. An endpoint that this port does not map at all
+	 * fails that requirement.
 	 */
 	if (pos < distance) {
 		dev_dbg(&cxlr->dev, "%s:%s: cannot host %s:%s at %d\n",
@@ -1378,7 +1379,7 @@ static int check_last_peer(struct cxl_endpoint_decoder *cxled,
 	cxled_peer = p->targets[pos - distance];
 	cxlmd_peer = cxled_to_memdev(cxled_peer);
 	ep_peer = cxl_ep_load(port, cxlmd_peer);
-	if (ep->dport != ep_peer->dport) {
+	if (!ep_peer || ep->dport != ep_peer->dport) {
 		dev_dbg(&cxlr->dev,
 			"%s:%s: %s:%s pos %d mismatched peer %s:%s\n",
 			dev_name(port->uport_dev), dev_name(&port->dev),

base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
-- 
2.37.3