[PATCH] reboot: validate reboot CPU against the possible mask

[Avi Weiss <thnkslprpt@gmail.com>]

reboot_setup() and cpu_store() compare the requested CPU ID against
num_possible_cpus(), which is a count rather than the highest possible
CPU ID. On sparse masks, this rejects valid high-numbered CPUs and
accepts impossible CPU IDs.

Validate the CPU ID against nr_cpu_ids and cpu_possible() instead.
Reject negative parsed values before the mask lookup and update the
error message to avoid assuming contiguous CPU numbering.

On an arm64 QEMU guest with possible CPUs 0,2-3, reboot=s3 selected
CPU 0 and sysfs rejected CPU 3 with -ERANGE. With the fix, reboot=s3
selects CPU 3 and sysfs accepts it, while both interfaces reject
impossible CPU 1.

Fixes: df5b0ab3e08a ("reboot: fix overflow parsing reboot cpu number")
Fixes: 2c622ed0eaa3 ("reboot: allow to specify reboot mode via sysfs")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---
 kernel/reboot.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index 9a84401adde5..7b09116376e3 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -1146,10 +1146,9 @@ static int __init reboot_setup(char *str)
 			if (isdigit(str[0])) {
 				int cpu = simple_strtoul(str, NULL, 0);
 
-				if (cpu >= num_possible_cpus()) {
-					pr_err("Ignoring the CPU number in reboot= option. "
-					"CPU %d exceeds possible cpu number %d\n",
-					cpu, num_possible_cpus());
+				if (cpu < 0 || cpu >= nr_cpu_ids || !cpu_possible(cpu)) {
+					pr_err("Ignoring invalid CPU %d in reboot= option\n",
+					       cpu);
 					break;
 				}
 				reboot_cpu = cpu;
@@ -1351,7 +1350,7 @@ static ssize_t cpu_store(struct kobject *kobj, struct kobj_attribute *attr,
 	if (rc)
 		return rc;
 
-	if (cpunum >= num_possible_cpus())
+	if (cpunum >= nr_cpu_ids || !cpu_possible(cpunum))
 		return -ERANGE;
 
 	reboot_default = 0;

base-commit: 36aad21d50d1af3f6954bb0c988257a908202477
-- 
2.43.0