[PATCH net v3] 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>
---
This patch fixes a race condition in netconsole's userdata handling
where concurrent message transmission could read partially updated
userdata fields, resulting in corrupted netconsole output.

The patch fixes the issue by ensuring update_userdata() holds
the target_list_lock while updating both extradata_complete and
userdata_length, preventing readers from seeing inconsistent state.

Changes in v3:
- Drop testcase.
- Link to v2: https://lore.kernel.org/r/20251022-netconsole-fix-race-v2-0-337241338079@meta.com

Changes in v2:
- Added testcase to Makefile.
- Reordered fix and testcase to avoid failure in CI.
- testcase: delay cleanup until child process are killed, plus shellcheck fixes.
- Link to v1: https://lore.kernel.org/all/20251020-netconsole-fix-race-v1-0-b775be30ee8a@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 194570443493..1f9cf6b12dfc 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,

---
base-commit: 84a905290cb4c3d9a71a9e3b2f2e02e031e7512f
change-id: 20251020-netconsole-fix-race-f465f37b57ea

Best regards,
-- 
Gustavo Duarte <gustavold@meta.com>