Re: [PATCH] random: add and use memzero_explicit() for clearing data

[Hannes Frederic Sowa]

On Mo, 2014-08-25 at 22:01 +0200, Daniel Borkmann wrote:
> zatimend has reported that in his environment (3.16/gcc4.8.3/corei7)
> memset() calls which clear out sensitive data in extract_{buf,entropy,
> entropy_user}() in random driver are being optimized away by gcc.
> 
> Add a helper memzero_explicit() (similarly as explicit_bzero() variants)
> that can be used in such cases where a variable with sensitive data is
> being cleared out in the end. Other use cases might also be in crypto
> code. [ I have put this into lib/string.c though, as it's always built-in
> and doesn't need any dependencies then. ]
> 
> Fixes kernel bugzilla: 82041
> 
> Reported-by: zatimend@xxxxxxxxxxxxx
> Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> Cc: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>

Acked-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>

In case this pattern of important function calls getting optimized away
emerges more often we could also go with a wrapper which forces the
execution of the function, like:

--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -181,6 +181,13 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 #define OPTIMIZER_HIDE_VAR(var) barrier()
 #endif
 
+#ifndef OPTIMIZER_FORCE_CALL
+#define OPTIMIZER_FORCE_CALL(func, args...) ({                         \
+                       typeof(func) * volatile __func = (func);        \
+                       __func(args);                                   \
+               })
+#endif
+
 /* Not-quite-unique ID. */
 #ifndef __UNIQUE_ID
 # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)


Thanks,
Hannes


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