[PATCH] mm: debug_page_alloc: fix NULL buf in debug_guardpage_minorder_setup

[Ye Liu <ye.liu@linux.dev>]

From: Ye Liu <liuye@kylinos.cn>

If the kernel command line includes "debug_guardpage_minorder" without
an equals sign (i.e., no value is provided), the early parameter
parser passes a NULL buf pointer to the setup function.

kstrtouint() does not perform a NULL check on its input and calls
directly into kstrtoull() which dereferences s[0] unconditionally,
leading to a NULL pointer dereference and early boot crash.

Additionally, the error path's pr_err("%s", buf) would also crash with
a NULL format argument.

Signed-off-by: Ye Liu <liuye@kylinos.cn>
---
 mm/debug_page_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/debug_page_alloc.c b/mm/debug_page_alloc.c
index 41e3d1f1ad96..fd2664c3c86a 100644
--- a/mm/debug_page_alloc.c
+++ b/mm/debug_page_alloc.c
@@ -22,8 +22,8 @@ static int __init debug_guardpage_minorder_setup(char *buf)
 {
 	unsigned int res;
 
-	if (kstrtouint(buf, 10, &res) < 0 ||  res > MAX_PAGE_ORDER / 2) {
-		pr_err("Bad debug_guardpage_minorder value: %s\n", buf);
+	if (!buf || kstrtouint(buf, 10, &res) < 0 || res > MAX_PAGE_ORDER / 2) {
+		pr_err("Bad debug_guardpage_minorder value: %s\n", buf ?: "(missing)");
 		return 0;
 	}
 	_debug_guardpage_minorder = res;
-- 
2.25.1