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

[Ho-Eun Ryu]

> On 30 Mar 2017, at 3:23 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
> 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)); \
> })
> 

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