printk: limit second loop of syslog_print_all

The second loop of syslog_print_all() subtracts lengths that were
added in the first loop. With commit b031a684 ("printk: remove
logbuf_lock writer-protection of ringbuffer") it is possible that
records are (over)written during syslog_print_all(). This allows the
possibility of the second loop subtracting lengths that were never
added in the first loop.

This situation can result in syslog_print_all() filling the buffer
starting from a later record, even though there may have been room
to fit the earlier record(s) as well.

Fixes: b031a684 ("printk: remove logbuf_lock writer-protection of ringbuffer")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-4-john.ogness@linutronix.de

kernel/printk/printk.c

@@ -1494,6 +1494,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	struct printk_info info;
 	unsigned int line_count;
 	struct printk_record r;
+	u64 max_seq;
 	char *text;
 	int len = 0;
 	u64 seq;
@@ -1512,9 +1513,15 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	prb_for_each_info(clear_seq, prb, seq, &info, &line_count)
 		len += get_record_print_text_size(&info, line_count, true, time);
 
+	/*
+	 * Set an upper bound for the next loop to avoid subtracting lengths
+	 * that were never added.
+	 */
+	max_seq = seq;
+
 	/* move first record forward until length fits into the buffer */
 	prb_for_each_info(clear_seq, prb, seq, &info, &line_count) {
-		if (len <= size)
+		if (len <= size || info.seq >= max_seq)
 			break;
 		len -= get_record_print_text_size(&info, line_count, true, time);
 	}