[BUG/PATCH] kernel RNG and its secrets

[mancha]

Hi.

The kernel RNG introduced memzero_explicit in d4c5efdb9777 to protect
memory cleansing against things like dead store optimization:

   void memzero_explicit(void *s, size_t count)
   {
           memset(s, 0, count);
           OPTIMIZER_HIDE_VAR(s);
   }

OPTIMIZER_HIDE_VAR, introduced in fe8c8a126806 to protect crypto_memneq
against timing analysis, is defined when using gcc as:

   #define OPTIMIZER_HIDE_VAR(var) __asm__ ("" : "=r" (var) : "0" (var))

My tests with gcc 4.8.2 on x86 find it insufficient to prevent gcc from
optimizing out memset (i.e. secrets remain in memory).

Two things that do work:

   __asm__ __volatile__ ("" : "=r" (var) : "0" (var))

   and

   __asm__ __volatile__("": : :"memory")

The first is OPTIMIZER_HIDE_VAR plus a volatile qualifier and the second
is barrier() [as defined when using gcc].

I propose memzero_explicit use barrier().

--- a/lib/string.c
+++ b/lib/string.c
@@ -616,7 +616,7 @@ EXPORT_SYMBOL(memset);
 void memzero_explicit(void *s, size_t count)
 {
        memset(s, 0, count);
-       OPTIMIZER_HIDE_VAR(s);
+       barrier();
 }
 EXPORT_SYMBOL(memzero_explicit);
 
For any attribution deemed necessary, please use "mancha security".
Please CC me on replies.

--mancha

PS CC'ing Herbert Xu in case this impacts crypto_memneq.

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