linux-kernel - Re: [PATCH next v3 0/5] minmax: Relax type checks in min() and max().

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Date:   Mon, 14 Aug 2023 14:21:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        'Andy Shevchenko' <andriy.shevchenko@...ux.intel.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        "'Matthew Wilcox (Oracle)'" <willy@...radead.org>,
        'Christoph Hellwig' <hch@...radead.org>,
        "'Jason A. Donenfeld'" <Jason@...c4.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH next v3 0/5] minmax: Relax type checks in min() and max().

On Fri, Aug 04, 2023 at 10:50:59AM +0000, David Laight wrote:
> [...]
> I also suspect that many of the min_t(u16, ...) are actually wrong.
> For example copy_data() in printk_ringbuffer.c contains:
>         data_size = min_t(u16, buf_size, len);
> Here buf_size is 'unsigned int' and len 'u16', pass a 64k buffer
> (can you prove that doesn't happen?) and no data is returned.

Stars aligning... this exact bug (as you saw in the other thread[1]) got
hit. And in the analysis, I came to the same conclusion: min_t() is a
serious foot-gun, and we should be able to make min() Just Work in the
most common situations.

It seems like the existing type_max/type_min macros could be used to
figure out that the args are safe to appropriately automatically cast,
etc. e.g. type_max(u16) <= type_max(unsigned int) && type_min(u16) >=
type_min(unsigned int) ...

-Kees

[1] https://lore.kernel.org/all/20230811054528.never.165-kees@kernel.org/

-- 
Kees Cook