[PATCH v3 8/9] perf/cxl: Validate the hardware-reported counter width

cxl_pmu_parse_caps() takes the Counter Width straight from the CPMU
Capability register without a bounds check. The Counter Data register is 64
bits wide, so a device reporting 0 or more than 64 is reporting nonsense,
and GENMASK_ULL(width - 1, 0) in the read path then shifts out of range.
That is undefined behaviour, and a UBSAN splat where it is enabled.

Reject a counter width outside 1..64 at probe.

Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
Reported-by: [email protected]
Closes: https://sashiko.dev/#/patchset/[email protected]?part=1
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Jonathan Cameron <[email protected]>
Signed-off-by: Dave Jiang <[email protected]>
---
v3:
- No change; picked up review tag.
---
 drivers/perf/cxl_pmu.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
index 580a75bc9210..37742ce43d9f 100644
--- a/drivers/perf/cxl_pmu.c
+++ b/drivers/perf/cxl_pmu.c
@@ -142,6 +142,15 @@ static int cxl_pmu_parse_caps(struct device *dev, struct cxl_pmu_info *info)
 
 	info->num_counters = FIELD_GET(CXL_PMU_CAP_NUM_COUNTERS_MSK, val) + 1;
 	info->counter_width = FIELD_GET(CXL_PMU_CAP_COUNTER_WIDTH_MSK, val);
+	/*
+	 * The Counter Data register is 64 bits wide, so a Counter Width of 0 or
+	 * >64 is invalid. Reject it rather than let GENMASK_ULL(width - 1, 0) in
+	 * the read path shift out of range.
+	 */
+	if (info->counter_width == 0 || info->counter_width > 64) {
+		dev_err(dev, "Invalid counter width %d\n", info->counter_width);
+		return -ENODEV;
+	}
 	info->num_event_capabilities = FIELD_GET(CXL_PMU_CAP_NUM_EVN_CAP_REG_SUP_MSK, val) + 1;
 
 	info->filter_hdm = FIELD_GET(CXL_PMU_CAP_FILTERS_SUP_MSK, val) & CXL_PMU_FILTER_HDM;
-- 
2.55.0