printk/console: Always disable boot consoles that use init memory before it is freed

Commit 4c30c6f5 ("kernel/printk: do not turn off bootconsole in
printk_late_init() if keep_bootcon") added a check on keep_bootcon to
ensure that boot consoles were kept around until the real console is
registered.

This can lead to problems if the boot console data and code are in the
init section, since it can be freed before the boot console is
unregistered.

Commit 81cc26f2 ("printk: only unregister boot consoles when
necessary") fixed this a better way. It allowed to keep boot consoles
that did not use init data. Unfortunately it did not remove the check
of keep_bootcon.

This can lead to crashes and weird panics when the bootconsole is
accessed after free, especially if page poisoning is in use and the
code / data have been overwritten with a poison value.

To prevent this, always free the boot console if it is within the init
section. In addition, print a warning about that the console is removed
prematurely.

Finally there is a new comment how to avoid the warning. It replaced
an explanation that duplicated a more comprehensive function
description few lines above.

Fixes: 4c30c6f5 ("kernel/printk: do not turn off bootconsole in printk_late_init() if keep_bootcon")
Link: http://lkml.kernel.org/r/1500036673-7122-2-git-send-email-pmladek@suse.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: "Fabio M. Di Nitto" <fdinitto@redhat.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
[pmladek@suse.com: print the warning, code and comments clean up]
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -2650,9 +2650,8 @@ void __init console_init(void)
  * makes it difficult to diagnose problems that occur during this time.
  *
  * To mitigate this problem somewhat, only unregister consoles whose memory
- * intersects with the init section. Note that code exists elsewhere to get
- * rid of the boot console as soon as the proper console shows up, so there
- * won't be side-effects from postponing the removal.
+ * intersects with the init section. Note that all other boot consoles will
+ * get unregistred when the real preferred console is registered.
  */
 static int __init printk_late_init(void)
 {
@@ -2660,15 +2659,14 @@ static int __init printk_late_init(void)
 	int ret;
 
 	for_each_console(con) {
-		if (!keep_bootcon && con->flags & CON_BOOT) {
+		if ((con->flags & CON_BOOT) &&
+		    init_section_intersects(con, sizeof(*con))) {
 			/*
-			 * Make sure to unregister boot consoles whose data
-			 * resides in the init section before the init section
-			 * is discarded. Boot consoles whose data will stick
-			 * around will automatically be unregistered when the
-			 * proper console replaces them.
+			 * Please, consider moving the reported consoles out
+			 * of the init section.
 			 */
-			if (init_section_intersects(con, sizeof(*con)))
+			pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
+				con->name, con->index);
 			unregister_console(con);
 		}
 	}