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

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Message-Id: <20251020-netconsole-fix-race-v1-2-b775be30ee8a@gmail.com>
Date: Mon, 20 Oct 2025 14:22:35 -0700
From: Gustavo Luiz Duarte <gustavold@...il.com>
To: Breno Leitao <leitao@...ian.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Shuah Khan <shuah@...nel.org>, Matthew Wood <thepacketgeek@...il.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, Gustavo Luiz Duarte <gustavold@...il.com>
Subject: [PATCH net 2/2] 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.

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

Signed-off-by: Gustavo Luiz Duarte <gustavold@...il.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