Commit 884e370e authored Sep 28, 2018 by Sergey Senozhatsky Committed by Petr Mladek Oct 02, 2018

printk: CON_PRINTBUFFER console registration is a bit racy

CON_PRINTBUFFER console registration requires us to do several
preparation steps:
- Rollback console_seq to replay logbuf messages which were already
  seen on other consoles;
- Set exclusive_console flag so console_unlock() will ->write() logbuf
  messages only to the exclusive_console driver.

The way we do it, however, is a bit racy

	logbuf_lock_irqsave(flags);
	console_seq = syslog_seq;
	console_idx = syslog_idx;
	logbuf_unlock_irqrestore(flags);
						<< preemption enabled
						<< irqs enabled
	exclusive_console = newcon;
	console_unlock();

We rollback console_seq under logbuf_lock with IRQs disabled, but
we set exclusive_console with local IRQs enabled and logbuf unlocked.
If the system oops-es or panic-s before we set exclusive_console - and
given that we have IRQs and preemption enabled there is such a
possibility - we will re-play all logbuf messages to every registered
console, which may be a bit annoying and time consuming.

Move exclusive_console assignment to the same IRQs-disabled and
logbuf_lock-protected section where we rollback console_seq.

Link: http://lkml.kernel.org/r/20180928095304.9972-1-sergey.senozhatsky@gmail.com


To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

parent f92b070f

kernel/printk/printk.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -2703,14 +2703,18 @@ void register_console(struct console *newcon)
		logbuf_lock_irqsave(flags);
		console_seq = syslog_seq;
		console_idx = syslog_idx;
		logbuf_unlock_irqrestore(flags);
		/*
		* We're about to replay the log buffer. Only do this to the
		* just-registered console to avoid excessive message spam to
		* the already-registered consoles.
		*
		* Set exclusive_console with disabled interrupts to reduce
		* race window with eventual console_flush_on_panic() that
		* ignores console_lock.
		*/
		exclusive_console = newcon;
		exclusive_console_stop_seq = console_seq;
		logbuf_unlock_irqrestore(flags);
		}
		console_unlock();
		console_sysfs_notify();

Admin message