On 07/31/2018 05:00 PM, Kees Cook wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>
> Add shift_overflow() helper to assist driver authors in ensuring that
> shift operations don't cause overflows or other odd conditions.
>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>
> [kees: tweaked comments and commit log, dropped unneeded assignment]
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> include/linux/overflow.h | 31 +++++++++++++++++++++++++++++++
> 1 file changed, 31 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 8712ff70995f..69fc366ce865 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -202,6 +202,37 @@
>
> #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
>
> +/** check_shift_overflow() - Calculate a left-shifted value and check overflow
> + *
> + * @a: Value to be shifted
> + * @b: How many bits left to shift
> + * @d: Pointer to where to store the result
> + *
> + * Computes *@d = (@a << @s)
@b)
> + *
> + * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
> + * make sense. Example conditions:
> + * - 'a << s' causes bits to be lost when stored in *d.
> + * - 's' is garbage (e.g. negative) or so large that the result of
> + * 'a << s' is guaranteed to be 0.
> + * - 'a' is negative.
> + * - 'a << s' sets the sign bit, if any, in '*d'.
> + *
> + * '*d' will hold the results of the attempted shift, but is not
> + * considered "safe for use" if false is returned.
> + */
> +#define check_shift_overflow(a, s, d) ({ \
> + typeof(a) _a = a; \
> + typeof(s) _s = s; \
> + typeof(d) _d = d; \
> + u64 _a_full = _a; \
> + unsigned int _to_shift = \
> + _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \
> + *_d = (_a_full << _to_shift); \
> + (_to_shift != _s || *_d < 0 || _a < 0 || \
> + (*_d >> _to_shift) != _a); \
> +})
> +
> /**
> * array_size() - Calculate size of 2-dimensional array.
> *
>
--
~Randy
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