riscv: hwprobe: Avoid uninitialized read in hwprobe_get_cpus()

When cpusetsize < cpumask_size(), hwprobe_get_cpus() did not fully
initialize its copy of the cpu mask, which could cause non-deterministic
results from the riscv_hwprobe syscall on a system with more than 8 CPUs
when the supplied cpu mask is empty.  Address this by fully initializing
the cpu mask.

Fixes: e178bf1 ("RISC-V: hwprobe: Introduce which-cpus flag")
Signed-off-by: Mark Harris <mark.hsj@gmail.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Michael Ellerman <mpe@kernel.org>
Link: https://patch.msgid.link/20260714003056.73707-1-mark.hsj@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: mark4o

arch/riscv/kernel/sys_hwprobe.c

@@ -450,6 +450,7 @@ static int hwprobe_get_cpus(struct riscv_hwprobe __user *pairs,
 	if (cpusetsize > cpumask_size())
 		cpusetsize = cpumask_size();
 
+	cpumask_clear(&cpus);
 	ret = copy_from_user(&cpus, cpus_user, cpusetsize);
 	if (ret)
 		return -EFAULT;