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

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Message-ID: <878s8ujnfg.fsf@jogness.linutronix.de>
Date:   Fri, 15 Jan 2021 15:22:03 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
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()

Hi Petr,

I noticed some minor errors in the commit log and comments...

On 2021-01-14, John Ogness <john.ogness@...utronix.de> wrote:
> Before commit b6cf8b3f3312 ("printk: add lockless ringbuffer"),
> msg_print_text()

The correct commit where the change was made is: 896fbe20b4e2333fb55
("printk: use the lockless ringbuffer")

> 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 msg_print_text() has been replaced by record_print_text(), which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")

Ditto for Fixes tag:

896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer")

> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  kernel/printk/printk.c | 35 +++++++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ffdd0dc7ec6d..73f9eae19f05 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1293,9 +1293,15 @@ static size_t info_print_prefix(const struct printk_info  *info, bool syslog,
>   *   - Add prefix for each line.
>   *   - Add the trailing newline that has been removed in vprintk_store().
>   *   - Drop truncated lines that do not longer fit into the buffer.
> + *   - Add a trailing newline.
> + *   - Add a string terminator.

I didn't realize that the newline was already listed (since it was out
of order). I recommend these comments changing to:

- Add prefix for each line.
- Drop truncated lines that no longer fit into the buffer.
- Add the trailing newline that has been removed in vprintk_store().
- Add a string terminator.

Can you fixup the patch for the commit or do you need a v2?

John Ogness