[PATCH net 2/2] netconsole: Fix race condition in between reader and writer of userdata

[Gustavo Luiz Duarte <gustavold@gmail.com>]

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.

Fixes: df03f830d099 ("net: netconsole: cache userdata formatted string in netconsole_target")

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
 drivers/net/netconsole.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 194570443493b..1f9cf6b12dfc5 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -888,6 +888,9 @@ static void update_userdata(struct netconsole_target *nt)
 {
 	int complete_idx = 0, child_count = 0;
 	struct list_head *entry;
+	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;
@@ -918,6 +921,8 @@ static void update_userdata(struct netconsole_target *nt)
 	}
 	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,

-- 
2.47.3