[PATCH] printk/console: Check consistent sequence number when handling race in console_unlock()

[Petr Mladek <pmladek@suse.com>]

The standard printk() tries to flush the message to the console
immediately. It tries to take the console lock. If the lock is
already taken then the current owner is responsible for flushing
even the new message.

There is a small race window between checking whether a new message is
available and releasing the console lock. It is solved by re-checking
the state after releasing the console lock. If the check is positive
then console_unlock() tries to take the lock again and process the new
message as well.

The commit 996e966640ddea7b535c ("printk: remove logbuf_lock") causes that
console_seq is not longer read atomically. As a result, the re-check might
be done with an inconsistent 64-bit index.

Solve it by using the last sequence number that has been checked under
the console lock. In the worst case, it will take the lock again only
to realized that the new message has already been proceed. But it
was possible even before.

Fixes: commit 996e966640ddea7b535c ("printk: remove logbuf_lock")
Cc: stable@vger.kernel.org # 5.13
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 142a58d124d9..87411084075e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2545,6 +2545,7 @@ void console_unlock(void)
 	bool do_cond_resched, retry;
 	struct printk_info info;
 	struct printk_record r;
+	u64 next_seq;
 
 	if (console_suspended) {
 		up_console_sem();
@@ -2654,8 +2655,10 @@ void console_unlock(void)
 			cond_resched();
 	}
 
-	console_locked = 0;
+	/* Get consistent value of the next-to-be-used sequence number. */
+	next_seq = console_seq;
 
+	console_locked = 0;
 	up_console_sem();
 
 	/*
@@ -2664,7 +2667,7 @@ void console_unlock(void)
 	 * there's a new owner and the console_unlock() from them will do the
 	 * flush, no worries.
 	 */
-	retry = prb_read_valid(prb, console_seq, NULL);
+	retry = prb_read_valid(prb, next_seq, NULL);
 	printk_safe_exit_irqrestore(flags);
 
 	if (retry && console_trylock())
-- 
2.26.2