linux-hardening - RE: [PATCH] printk: ringbuffer: Fix truncating buffer size min

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Date:   Mon, 14 Aug 2023 10:42:26 +0000
From:   David Laight <David.Laight@...LAB.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>,
        "Vijay Balakrishna" <vijayb@...ux.microsoft.com>,
        "stable@...r.kernel.org" <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-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: RE: [PATCH] printk: ringbuffer: Fix truncating buffer size min_t cast

From: Kees Cook
> Sent: 11 August 2023 06:46
> 
> 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.
> 
...
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> index 2dc4d5a1f1ff..fde338606ce8 100644
> --- a/kernel/printk/printk_ringbuffer.c
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring,
>  	if (!buf || !buf_size)
>  		return true;
> 
> -	data_size = min_t(u16, buf_size, len);
> +	data_size = min_t(unsigned int, buf_size, len);

I'd noticed that during one of my test compiles while looking
at making min() less fussy.

A better fix would be:
	data_size = min(buf_size + 0u, len);

Or put an ack on my patch 3/5 to minmax.h and then min(buf_size, len)
will be fine (because both arguments are unsigned).

	David

> 
>  	memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
>  	return true;
> --
> 2.34.1

-
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