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

[Kees Cook]

On December 3, 2021 4:30:35 AM PST, Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>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?

There's an argument to be made that the first arg should be void * but that's a pretty invasive change at this point (and orthogonal to this fix).

I'd be happy to send a treewide change for that too, if folks wanted?

>
>> 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?

There should be none. It's entirely using constant expressions, so all of it gets reduce at compile time to a single path without conditionals. The spot checks I did on the machine code showed no differences either (since I think optimization was doing the masking vis smaller width dereference).


>

-- 
Kees Cook