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

On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook [off-list ref] 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