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From: Hannes Frederic Sowa <hannes@stressinduktion.org>
To: Daniel Borkmann <dborkman@redhat.com>
Cc: tytso@mit.edu, zatimend@hotmail.co.uk,
	linux-kernel@vger.kernel.org,
	Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH] random: add and use memzero_explicit() for clearing data
Date: Tue, 26 Aug 2014 01:11:30 +0200	[thread overview]
Message-ID: <1409008290.6274.73.camel@localhost> (raw)
In-Reply-To: <1408996899-4892-1-git-send-email-dborkman@redhat.com>

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@hotmail.co.uk
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

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



  parent reply	other threads:[~2014-08-25 23:11 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-08-25 20:01 [PATCH] random: add and use memzero_explicit() for clearing data Daniel Borkmann
2014-08-25 20:35 ` Alexey Dobriyan
2014-08-25 20:38   ` Daniel Borkmann
2014-08-25 23:11 ` Hannes Frederic Sowa [this message]
2014-08-27  3:21   ` Theodore Ts'o
2014-12-01 10:27 ` Dan Carpenter
2014-12-01 11:04   ` Daniel Borkmann
2014-12-01 11:29     ` Dan Carpenter
2014-12-01 11:38       ` Dan Carpenter
2014-12-01 13:39         ` Daniel Borkmann
2014-12-01 11:48       ` Daniel Borkmann

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