netconsole: Fix race condition in between reader and writer of userdata

The update_userdata() function constructs the complete userdata string
in nt->extradata_complete and updates nt->userdata_length. This data
is then read by write_msg() and write_ext_msg() when sending netconsole
messages. However, update_userdata() was not holding target_list_lock
during this process, allowing concurrent message transmission to read
partially updated userdata.

This race condition could result in netconsole messages containing
incomplete or inconsistent userdata - for example, reading the old
userdata_length with new extradata_complete content, or vice versa,
leading to truncated or corrupted output.

Fix this by acquiring target_list_lock with spin_lock_irqsave() before
updating extradata_complete and userdata_length, and releasing it after
both fields are fully updated. This ensures that readers see a
consistent view of the userdata, preventing corruption during concurrent
access.

The fix aligns with the existing locking pattern used throughout the
netconsole code, where target_list_lock protects access to target
fields including buf[] and msgcounter that are accessed during message
transmission.

Also get rid of the unnecessary variable complete_idx, which makes it
easier to bail out of update_userdata().

Fixes: df03f830d099 ("net: netconsole: cache userdata formatted string in netconsole_target")
Signed-off-by: Gustavo Luiz Duarte <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/netconsole.c

···
 
 static void update_userdata(struct netconsole_target *nt)
 {
-	int complete_idx = 0, child_count = 0;
 	struct list_head *entry;
+	int child_count = 0;
+	unsigned long flags;
+
+	spin_lock_irqsave(&target_list_lock, flags);
 
 	/* Clear the current string in case the last userdatum was deleted */
 	nt->userdata_length = 0;
···
 		struct userdatum *udm_item;
 		struct config_item *item;
 
-		if (WARN_ON_ONCE(child_count >= MAX_EXTRADATA_ITEMS))
-			break;
+		if (child_count >= MAX_EXTRADATA_ITEMS) {
+			spin_unlock_irqrestore(&target_list_lock, flags);
+			WARN_ON_ONCE(1);
+			return;
+		}
 		child_count++;
 
 		item = container_of(entry, struct config_item, ci_entry);
···
 		 * one entry length (1/MAX_EXTRADATA_ITEMS long), entry count is
 		 * checked to not exceed MAX items with child_count above
 		 */
-		complete_idx += scnprintf(&nt->extradata_complete[complete_idx],
-					  MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n",
-					  item->ci_name, udm_item->value);
+		nt->userdata_length += scnprintf(&nt->extradata_complete[nt->userdata_length],
+						 MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n",
+						 item->ci_name, udm_item->value);
 	}
-	nt->userdata_length = strnlen(nt->extradata_complete,
-				      sizeof(nt->extradata_complete));
+	spin_unlock_irqrestore(&target_list_lock, flags);
 }
 
 static ssize_t userdatum_value_store(struct config_item *item, const char *buf,