Subject: [PATCH 12/48] printk: nbcon: Use driver synchronization while registering
From: John Ogness <[email-masked]>
Date: Fri, 15 Mar 2024 15:38:22 +0000

Depending on if an nbcon console is registered, a driver may
handle its internal locking differently. If a driver is holding
its internal lock while the nbcon console is registered, there
may be a risk that two different contexts access the hardware
simultaneously without synchronization. (For example, if the
printk subsystem invokes atomic printing while another driver
context acquired the internal lock without considering the
atomic console.)

Use the driver synchronization while a registering nbcon console
transitions to being registered. This guarantees that if the
driver acquires its internal lock when the nbcon console was not
registered, it will remain unregistered until that context
releases the lock.

Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/printk/printk.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

Index: linux-6.8.2-rt10/kernel/printk/printk.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/printk.c
+++ linux-6.8.2-rt10/kernel/printk/printk.c
@@ -3453,6 +3453,7 @@ void register_console(struct console *ne
 	struct console *con;
 	bool bootcon_registered = false;
 	bool realcon_registered = false;
+	unsigned long flags;
 	int err;
 
 	console_list_lock();
@@ -3545,6 +3546,19 @@ void register_console(struct console *ne
 	}
 
 	/*
+	 * If another context is actively using the hardware of this new
+	 * console, it will not be aware of the nbcon synchronization. This
+	 * is a risk that two contexts could access the hardware
+	 * simultaneously if this new console is used for atomic printing
+	 * and the other context is still using the hardware.
+	 *
+	 * Use the driver synchronization to ensure that the hardware is not
+	 * in use while this new console transitions to being registered.
+	 */
+	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
+		newcon->device_lock(newcon, &flags);
+
+	/*
 	 * Put this console in the list - keep the
 	 * preferred driver at the head of the list.
 	 */
@@ -3568,6 +3582,10 @@ void register_console(struct console *ne
 	 * register_console() completes.
 	 */
 
+	/* This new console is now registered. */
+	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
+		newcon->device_unlock(newcon, flags);
+
 	console_sysfs_notify();
 
 	/*