[ Cc'ing Cesar ]
On 03/18/2015 10:53 AM, mancha wrote:
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).
Could you elaborate on your test case?
memzero_explicit() is actually an EXPORT_SYMBOL(), are you saying
that gcc removes the call to memzero_explicit() entirely, inlines
it, and then optimizes the memset() eventually away?
Last time I looked, it emitted a call to memzero_explicit(), and
inside memzero_explicit() it did the memset() as it cannot make
any assumption from there. I'm using gcc (GCC) 4.8.3 20140911
(Red Hat 4.8.3-7).
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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