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

[Andy Shevchenko]

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