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

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Date:   Tue, 19 Jan 2021 12:20:04 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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 Tue 2021-01-19 09:44:56, Sergey Senozhatsky wrote:
> On (21/01/15 13:07), Petr Mladek wrote:
> > On Fri 2021-01-15 13:04:37, Petr Mladek wrote:
> > > 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 msg_print_text() has been replaced by record_print_text(), which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>
> 
> John, how did you spot these problems?
> 
> FWIW, Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

The patch has been committed into printk/linux.git, branch
printk-rework.

Best Regards,
Petr