Messages in this thread | | | Date | Fri, 03 Dec 2021 08:37:59 -0800 | From | Kees Cook <> | Subject | Re: [PATCH] find: Do not read beyon d variable boundaries on small sizes |
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On December 3, 2021 4:30:35 AM PST, Andy Shevchenko <andriy.shevchenko@linux.intel.com> 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).
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-- Kees Cook
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