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

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Date:   Fri, 3 Dec 2021 14:30:35 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Yury Norov <yury.norov@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        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 Fri, Dec 03, 2021 at 02:08:46AM -0800, Kees Cook wrote:
> It's common practice to cast small variable arguments to the find_*_bit()

It's a bad practice and should be fixed accordingly, no?

> 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);
> 
> This leads to the find helper dereferencing a full unsigned long,
> regardless of the size of the actual variable. The unwanted bits
> get masked away, but strictly speaking, a read beyond the end of
> the target variable happens. Builds under -Warray-bounds complain
> about this situation, for example:
> 
> In file included from ./include/linux/bitmap.h:9,
>                  from drivers/iommu/intel/iommu.c:17:
> drivers/iommu/intel/iommu.c: In function 'domain_context_mapping_one':
> ./include/linux/find.h:119:37: error: array subscript 'long unsigned int[0]' is partly outside array bounds of 'int[1]' [-Werror=array-bounds]
>   119 |                 unsigned long val = *addr & GENMASK(size - 1, 0);
>       |                                     ^~~~~
> drivers/iommu/intel/iommu.c:2115:18: note: while referencing 'max_pde'
>  2115 |         int pds, max_pde;
>       |                  ^~~~~~~
> 
> Instead, just carefully read the correct variable size, all of which
> happens at compile time since small_const_nbits(size) has already
> determined that arguments are constant expressions.

What is the performance impact?

-- 
With Best Regards,
Andy Shevchenko