fs/filesystems: Fix potential unsigned integer underflow in fs_name()

[ Upstream commit 1363c13 ]

fs_name() has @index as unsigned int, so there is underflow risk for
operation '@index--'.

Fix by breaking the for loop when '@index == 0' which is also more proper
than '@index <= 0' for unsigned integer comparison.

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/20250410-fix_fs-v1-1-7c14ccc8ebaa@quicinc.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: zijun-hu

fs/filesystems.c

@@ -156,15 +156,19 @@ static int fs_index(const char __user * __name)
 static int fs_name(unsigned int index, char __user * buf)
 {
 	struct file_system_type * tmp;
-	int len, res;
+	int len, res = -EINVAL;
 
 	read_lock(&file_systems_lock);
-	for (tmp = file_systems; tmp; tmp = tmp->next, index--)
-		if (index <= 0 && try_module_get(tmp->owner))
+	for (tmp = file_systems; tmp; tmp = tmp->next, index--) {
+		if (index == 0) {
+			if (try_module_get(tmp->owner))
+				res = 0;
 			break;
+		}
+	}
 	read_unlock(&file_systems_lock);
-	if (!tmp)
-		return -EINVAL;
+	if (res)
+		return res;
 
 	/* OK, we got the reference, so we can safely block */
 	len = strlen(tmp->name) + 1;