Re: [PATCH] printk: fix buffer overflow potential for print_text()

[Petr Mladek <pmladek@suse.com>]

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 msg_print_text() has been replaced by record_print_text(), which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@linutronix.de>

I forgot one thing. We should add stable here:

Cc: stable@vger.kernel.org # 5.10+

No need to resend the patch. I'll add it when pushing the patch.

Best Regards,
Petr