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

On 30 Mar 2017, at 3:23 AM, Kees Cook [off-list ref] wrote:

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

quoted

+/*
+ * 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)); \
})

Right, and it has a problem with BUILD_BUG, which causes compilation error when CONFIG_HABE_ARCH_RARE_WRITE_MEMCPY is true
BUILD_BUG is defined in <linux/bug.h> but <linux/bug.h> includes <linux/compiler.h>

Please see the following.

diff --git a/include/linux/compiler.h b/include/linux/compiler.h                
index 3334fa9..3fa50e1 100644                                                   
--- a/include/linux/compiler.h                                                  
+++ b/include/linux/compiler.h                                                  

@@ -350,11 +350,11 @@ static __always_inline void __write_once_size(volatile vo\

id *p, void *res, int s
 # 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_n(var, val, len)        ({                                 \                                                                              
+               typeof(val) __val = val;                                                    \
+               __arch_rare_write_memcpy(&(var), &(__val), (len));      \       
        })
-#  define __rare_write(var, val)  __rare_write_n(&(var), &(val), sizeof(var))  
+#  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                                                                        

-Kees

-- 
Kees Cook
Pixel Security