[PATCH printk v2] printk: Avoid multiple unregistration in unregister_console_locked()

[John Ogness <john.ogness@linutronix.de>]

unregister_console_locked() is called from within
hlist_for_each_entry_safe(console_list) loops. This would be safe if
unregister_console_locked() only unregistered the specified console. After
all, that is what the _safe variant is for.

However, unregister_console_locked() calls printk_kthreads_check_locked(),
which can unregister _multiple_ consoles if kthreads fail to start. This
could lead to hlist_for_each_entry_safe() use-after-free scenarios because
not only was the list entry removed, but possibly the following entry as
well (the @n temporary storage of the next entry).

Move the call to printk_kthreads_check_locked() outside of
unregister_console_locked(). Add comments to both functions to clarify
if they are safe to be used in hlist_for_each_entry_safe(console_list)
loops.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/lkml/87ik31iz4x.fsf@jogness.linutronix.de
Fixes: 76f258bf3f2a ("printk: nbcon: Introduce printer kthreads")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
v1: https://lore.kernel.org/lkml/20261029152454.638952-1-john.ogness@linutronix.de

changes since v1:

- In printk_late_init(), move printk_kthreads_check_locked() outside of
  the hlist_for_each_entry_safe(console_list) loop.

 kernel/printk/printk.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9a205f94bfac7..5dbcf8a9909bb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3792,6 +3792,13 @@ static struct syscore printk_syscore = {
  * If appropriate, start nbcon kthreads and set @printk_kthreads_running.
  * If any kthreads fail to start, those consoles are unregistered.
  *
+ * Called after consoles are registered or unregistered.
+ *
+ * This MUST NOT be called within an hlist_for_each_entry_safe(console_list)
+ * loop because it can unregister multiple consoles if any kthreads fail to
+ * start. That would lead to use-after-free scenarios within the
+ * hlist_for_each_entry_safe(console_list) loop of the caller.
+ *
  * Must be called under console_list_lock().
  */
 static void printk_kthreads_check_locked(void)
@@ -4230,7 +4237,20 @@ void register_console(struct console *newcon)
 }
 EXPORT_SYMBOL(register_console);
 
-/* Must be called under console_list_lock(). */
+/*
+ * Unregister the specified console.
+ *
+ * This can be called safely from within an
+ * hlist_for_each_entry_safe(console_list) loop because it unregisters
+ * _only_ the specified console.
+ *
+ * Afterwards the caller should call printk_kthreads_check_locked() since
+ * unregistering a console may require printer threads to start/stop. Note
+ * that printk_kthreads_check_locked() cannot be called safely from within
+ * an hlist_for_each_entry_safe(console_list) loop.
+ *
+ * Must be called under console_list_lock().
+ */
 static int unregister_console_locked(struct console *console)
 {
 	bool use_device_lock = (console->flags & CON_NBCON) && console->write_atomic;
@@ -4322,9 +4342,6 @@ static int unregister_console_locked(struct console *console)
 	if (console->exit)
 		res = console->exit(console);
 
-	/* Changed console list, may require printer threads to start/stop. */
-	printk_kthreads_check_locked();
-
 	return res;
 }
 
@@ -4333,7 +4350,12 @@ int unregister_console(struct console *console)
 	int res;
 
 	console_list_lock();
+
 	res = unregister_console_locked(console);
+
+	/* Changed console list, may require printer threads to start/stop. */
+	printk_kthreads_check_locked();
+
 	console_list_unlock();
 	return res;
 }
@@ -4472,6 +4494,10 @@ static int __init printk_late_init(void)
 
 		unregister_console_locked(con);
 	}
+
+	/* Changed console list, may require printer threads to start/stop. */
+	printk_kthreads_check_locked();
+
 	console_list_unlock();
 
 	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,

base-commit: 647ef123d19695b5d8c79e03c86fb17c558e30b7
-- 
2.47.3