linux-kernel - Re: [PATCH] printk: fix buffer overflow potential for print

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Message-ID: <YAGE1O/nG57hyRs4@alley>
Date:   Fri, 15 Jan 2021 13:04:36 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: fix buffer overflow potential for print_text()

On Thu 2021-01-14 18:10:12, John Ogness wrote:
> Before commit b6cf8b3f3312 ("printk: add lockless ringbuffer"),
> msg_print_text() would only write up to size-1 bytes into the
> provided buffer. Some callers expect this behavior and append
> a terminator to returned string. In particular:
> 
> arch/powerpc/xmon/xmon.c:dump_log_buf()
> arch/um/kernel/kmsg_dump.c:kmsg_dumper_stdout()
> 
> msg_print_text() has been replaced by record_print_text(), which
> currently fills the full size of the buffer. This causes a
> buffer overflow for the above callers.
> 
> Change record_print_text() so that it will only use size-1 bytes
> for text data. Also, for paranoia sakes, add a terminator after
> the text data.
> 
> And finally, document this behavior so that it is clear that only
> size-1 bytes are used and a terminator is added.
> 
> Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr