nvmet: Don't overflow subsysnqn

nvmet_root_discovery_nqn_store treats the subsysnqn string like a fixed
size buffer, even though it is dynamically allocated to the size of the
string.

Create a new string with kstrndup instead of using the old buffer.

Reported-by: syzbot+ff4aab278fa7e27e0f9e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ff4aab278fa7e27e0f9e
Fixes: 95409e2 ("nvmet: implement unique discovery NQN")
Signed-off-by: Leo Stone <leocstone@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/target/configfs.c

@@ -2254,12 +2254,17 @@ static ssize_t nvmet_root_discovery_nqn_store(struct config_item *item,
 		const char *page, size_t count)
 {
 	struct list_head *entry;
+	char *old_nqn, *new_nqn;
 	size_t len;
 
 	len = strcspn(page, "\n");
 	if (!len || len > NVMF_NQN_FIELD_LEN - 1)
 		return -EINVAL;
 
+	new_nqn = kstrndup(page, len, GFP_KERNEL);
+	if (!new_nqn)
+		return -ENOMEM;
+
 	down_write(&nvmet_config_sem);
 	list_for_each(entry, &nvmet_subsystems_group.cg_children) {
 		struct config_item *item =
@@ -2268,13 +2273,15 @@ static ssize_t nvmet_root_discovery_nqn_store(struct config_item *item,
 		if (!strncmp(config_item_name(item), page, len)) {
 			pr_err("duplicate NQN %s\n", config_item_name(item));
 			up_write(&nvmet_config_sem);
+			kfree(new_nqn);
 			return -EINVAL;
 		}
 	}
-	memset(nvmet_disc_subsys->subsysnqn, 0, NVMF_NQN_FIELD_LEN);
-	memcpy(nvmet_disc_subsys->subsysnqn, page, len);
+	old_nqn = nvmet_disc_subsys->subsysnqn;
+	nvmet_disc_subsys->subsysnqn = new_nqn;
 	up_write(&nvmet_config_sem);
 
+	kfree(old_nqn);
 	return len;
 }