Re: [RFC v2][PATCH 01/11] Introduce rare_write() infrastructure

[Kees Cook]

On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> +/*
> + * Build "write rarely" infrastructure for flipping memory r/w
> + * on a per-CPU basis.
> + */
> +#ifndef CONFIG_HAVE_ARCH_RARE_WRITE
> +# define __wr_rare
> +# define __wr_rare_type
> +# define __rare_write(__var, __val)    (__var = (__val))
> +# define rare_write_begin()            do { } while (0)
> +# define rare_write_end()              do { } while (0)
> +#else
> +# define __wr_rare                     __ro_after_init
> +# define __wr_rare_type                        const
> +# ifdef CONFIG_HAVE_ARCH_RARE_WRITE_MEMCPY
> +#  define __rare_write_n(dst, src, len)        ({                      \
> +               BUILD_BUG(!builtin_const(len));                 \
> +               __arch_rare_write_memcpy((dst), (src), (len));  \
> +       })
> +#  define __rare_write(var, val)  __rare_write_n(&(var), &(val), sizeof(var))
> +# else
> +#  define __rare_write(var, val)  ((*(typeof((typeof(var))0) *)&(var)) = (val))
> +# endif
> +# define rare_write_begin()    __arch_rare_write_begin()
> +# define rare_write_end()      __arch_rare_write_end()
> +#endif
> +#define rare_write(__var, __val) ({                    \
> +       rare_write_begin();                             \
> +       __rare_write(__var, __val);                     \
> +       rare_write_end();                               \
> +       __var;                                          \
> +})
> +

Of course, only after sending this do I realize that the MEMCPY case
will need to be further adjusted, since it currently can't take
literals. I guess something like this needs to be done:

#define __rare_write(var, val) ({ \
    typeof(var) __src = (val);     \
    __rare_write_n(&(var), &(__src), sizeof(var)); \
})

-Kees

-- 
Kees Cook
Pixel Security