linux-kernel - Re: [PATCH] printk: ringbuffer: Fix truncating buffer size min

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Date:   Thu, 10 Aug 2023 23:16:05 -0700
From:   Vijay Balakrishna <vijayb@...ux.microsoft.com>
To:     Kees Cook <keescook@...omium.org>, Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        stable@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] printk: ringbuffer: Fix truncating buffer size min_t cast

On 8/10/23 22:45, Kees Cook wrote:
> If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in
> copy_data() was causing writes to truncate. This manifested as output
> bytes being skipped, seen as %NUL bytes in pstore dumps when the available
> record size was larger than 65536. Fix the cast to no longer truncate
> the calculation.
> 
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: John Ogness <john.ogness@...utronix.de>
> Reported-by: Vijay Balakrishna <vijayb@...ux.microsoft.com>
> Closes: https://lore.kernel.org/lkml/d8bb1ec7-a4c5-43a2-9de0-9643a70b899f@linux.microsoft.com/
> Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Excellent.

I have verified on v5.10.

Tested-by: Vijay Balakrishna <vijayb@...ux.microsoft.com>

Thanks,
Vijay