[PATCH v2 9/9] perf/cxl: Avoid cpumask_of(-1) when no CPU is assigned

cpumask_show() feeds info->on_cpu straight into cpumask_of() for the
world-readable cpumask sysfs attribute. on_cpu is -1 before the first
hotplug online callback and transiently in cxl_pmu_offline_cpu() before a
new target is chosen. cpumask_of(-1) treats the CPU number as unsigned and
does out-of-bounds pointer arithmetic in get_cpu_mask(), so a concurrent
read of the attribute dereferences a wild pointer and can fault -- a local
denial of service.

Read on_cpu once and emit an empty mask when it is negative.

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
Signed-off-by: Dave Jiang <[email protected]>
---
 drivers/perf/cxl_pmu.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
index f42238b2b6b0..6aad381c0376 100644
--- a/drivers/perf/cxl_pmu.c
+++ b/drivers/perf/cxl_pmu.c
@@ -501,8 +501,17 @@ static ssize_t cpumask_show(struct device *dev, struct device_attribute *attr,
 			    char *buf)
 {
 	struct cxl_pmu_info *info = dev_get_drvdata(dev);
+	int cpu = READ_ONCE(info->on_cpu);
 
-	return cpumap_print_to_pagebuf(true, buf, cpumask_of(info->on_cpu));
+	/*
+	 * on_cpu is -1 before the first online callback and transiently during
+	 * cxl_pmu_offline_cpu(). cpumask_of(-1) computes an out-of-bounds
+	 * pointer, so report an empty mask instead.
+	 */
+	if (cpu < 0)
+		return sysfs_emit(buf, "\n");
+
+	return cpumap_print_to_pagebuf(true, buf, cpumask_of(cpu));
 }
 static DEVICE_ATTR_RO(cpumask);
 
-- 
2.55.0