linux-hardening - Re: [PATCH] find: Do not read beyond variable boundaries on small sizes

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Message-ID: <037c44cb-c5bc-9a9a-0c16-219bdf858ea7@rasmusvillemoes.dk>
Date:   Thu, 9 Dec 2021 00:23:10 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kees Cook <keescook@...omium.org>,
        Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] find: Do not read beyond variable boundaries on small
 sizes

On 03/12/2021 11.08, Kees Cook wrote:
> It's common practice to cast small variable arguments to the find_*_bit()
> helpers to unsigned long and then use a size argument smaller than
> sizeof(unsigned long):
> 
> 	unsigned int bits;
> 	...
> 	out = find_first_bit((unsigned long *)&bits, 32);

Those call sites need to be fixed, they are broken on BE anyway. And
your __find_bits_deref does nothing to fix (paper over) that if, say,
the caller uses an u32 to store an 8-bit bitmap.

So NAK.

Rasmus