[PATCH] sched/numa: fix division by zero in task_nr_scan_windows() and task_scan_min()

[Xiang Gao <gxxa03070307@gmail.com>]

From: Xiang Gao <gaoxiang17@xiaomi.com>

sysctl_numa_balancing_scan_size is exposed to userspace via debugfs as
scan_size_mb with no lower-bound validation. Writing 0 triggers
division-by-zero crashes in two places:

  task_nr_scan_windows():
    nr_scan_pages = MB_TO_PAGES(0) = 0
    rss = round_up(rss, 0)    <- undefined
    return rss / 0             <- division by zero

  task_scan_min():
    windows = MAX_SCAN_WINDOW / 0  <- division by zero

The third call site in task_numa_work() already guards against this
with an explicit "if (!pages) return;" check at line 3522, but these
two functions lack the same protection.

Fix by clamping scan_size to a minimum of 1 in both functions before
using it as a divisor, consistent with the existing guard in
task_numa_work().

Signed-off-by: Xiang Gao <gaoxiang17@xiaomi.com>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69361c63353a..32e146796bd5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1665,6 +1665,8 @@ static unsigned int task_nr_scan_windows(struct task_struct *p)
 	 * on resident pages
 	 */
 	nr_scan_pages = MB_TO_PAGES(sysctl_numa_balancing_scan_size);
+	if (!nr_scan_pages)
+		nr_scan_pages = 1;
 	rss = get_mm_rss(p->mm);
 	if (!rss)
 		rss = nr_scan_pages;
@@ -1682,6 +1684,8 @@ static unsigned int task_scan_min(struct task_struct *p)
 	unsigned int scan, floor;
 	unsigned int windows = 1;
 
+	if (!scan_size)
+		scan_size = 1;
 	if (scan_size < MAX_SCAN_WINDOW)
 		windows = MAX_SCAN_WINDOW / scan_size;
 	floor = 1000 / windows;
-- 
2.34.1