[PATCH] cxl/mbox: Break poison list loop on an empty payload

[Dave Jiang <dave.jiang@intel.com>]

A device that returns count == 0 with CXL_POISON_FLAG_MORE set on every
iteration never advances nr_records, so the max_errors guard never
trips and the do/while loops forever while holding poison.mutex. That
hangs the sysfs-triggered scan thread and blocks all subsequent poison
operations on the device. The existing "Protect against an uncleared
_FLAG_MORE" guard was intended to bound a misbehaving device but does
not cover the count == 0 case.

Stop the loop on an empty payload so a malfunctioning or malicious
device cannot wedge the poison scan.

Link: https://sashiko.dev/#/patchset/20260702090849.47501-1-icheng@nvidia.com?part=3
Fixes: ed83f7ca398b ("cxl/mbox: Add GET_POISON_LIST mailbox command")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
 drivers/cxl/core/mbox.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
index 7c6c5b7450a5..c09fae2cae9f 100644
--- a/drivers/cxl/core/mbox.c
+++ b/drivers/cxl/core/mbox.c
@@ -1455,6 +1455,11 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
 		if (rc)
 			break;
 
+		if (!le16_to_cpu(po->count)) {
+			dev_dbg(&cxlmd->dev, "Poison empty payload!\n");
+			break;
+		}
+
 		for (int i = 0; i < le16_to_cpu(po->count); i++)
 			trace_cxl_poison(cxlmd, cxlr, &po->record[i],
 					 po->flags, po->overflow_ts,

base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
-- 
2.54.0