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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