linux-kernel - Re: [PATCH 1/2] printk/console: Always disable boot consoles that use init memory before it is freed

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Date:   Wed, 26 Jul 2017 22:07:31 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Redfearn <matt.redfearn@...tec.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "Fabio M. Di Nitto" <fdinitto@...hat.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 1/2] printk/console: Always disable boot consoles that
 use init memory before it is freed

On (07/14/17 14:51), Petr Mladek wrote:
> From: Matt Redfearn <matt.redfearn@...tec.com>
> 
> Commit 4c30c6f566c0 ("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 81cc26f2bd11 ("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: 4c30c6f566c0 ("kernel/printk: do not turn off bootconsole in printk_late_init() if keep_bootcon")
> Signed-off-by: Matt Redfearn <matt.redfearn@...tec.com>
> [pmladek@...e.com: print the warning, code and comments clean up]
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

	-ss