[PATCH 4.19 056/220] printk: CON_PRINTBUFFER console registration is a bit racy

From: Greg Kroah-Hartman
Date: Fri Nov 22 2019 - 06:12:16 EST

From: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>

[ Upstream commit 884e370ea88c109a3b982f4eb9ecd82510a3a1fe ]

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

console_seq = syslog_seq;
console_idx = syslog_idx;
<< preemption enabled
<< irqs enabled
exclusive_console = newcon;

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@xxxxxxxxx
To: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
kernel/printk/printk.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b627954061bb6..11d70fd15e706 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2708,14 +2708,18 @@ void register_console(struct console *newcon)
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);