diff options
| -rw-r--r-- | src/support/cleanse.cpp | 32 | ||||
| -rw-r--r-- | src/support/cleanse.h | 3 |
2 files changed, 14 insertions, 21 deletions
diff --git a/src/support/cleanse.cpp b/src/support/cleanse.cpp index f895e96568..ecb00510f7 100644 --- a/src/support/cleanse.cpp +++ b/src/support/cleanse.cpp @@ -11,33 +11,25 @@ #include <Windows.h> // For SecureZeroMemory. #endif -/* Compilers have a bad habit of removing "superfluous" memset calls that - * are trying to zero memory. For example, when memset()ing a buffer and - * then free()ing it, the compiler might decide that the memset is - * unobservable and thus can be removed. - * - * Previously we used OpenSSL which tried to stop this by a) implementing - * memset in assembly on x86 and b) putting the function in its own file - * for other platforms. - * - * This change removes those tricks in favour of using asm directives to - * scare the compiler away. As best as our compiler folks can tell, this is - * sufficient and will continue to be so. - * - * Adam Langley <agl@google.com> - * Commit: ad1907fe73334d6c696c8539646c21b11178f20f - * BoringSSL (LICENSE: ISC) - */ void memory_cleanse(void *ptr, size_t len) { #if defined(_MSC_VER) + /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */ SecureZeroMemory(ptr, len); #else std::memset(ptr, 0, len); - /* As best as we can tell, this is sufficient to break any optimisations that - might try to eliminate "superfluous" memsets. If there's an easy way to - detect memset_s, it would be better to use that. */ + /* Memory barrier that scares the compiler away from optimizing out the memset. + * + * Quoting Adam Langley <agl@google.com> in commit ad1907fe73334d6c696c8539646c21b11178f20f + * in BoringSSL (ISC License): + * As best as we can tell, this is sufficient to break any optimisations that + * might try to eliminate "superfluous" memsets. + * This method is used in memzero_explicit() the Linux kernel, too. Its advantage is that it + * is pretty efficient because the compiler can still implement the memset() efficiently, + * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by + * Yang et al. (USENIX Security 2017) for more background. + */ __asm__ __volatile__("" : : "r"(ptr) : "memory"); #endif } diff --git a/src/support/cleanse.h b/src/support/cleanse.h index 5298214e44..b03520315d 100644 --- a/src/support/cleanse.h +++ b/src/support/cleanse.h @@ -8,7 +8,8 @@ #include <stdlib.h> -// Attempt to overwrite data in the specified memory span. +/** Secure overwrite a buffer (possibly containing secret data) with zero-bytes. The write + * operation will not be optimized out by the compiler. */ void memory_cleanse(void *ptr, size_t len); #endif // BITCOIN_SUPPORT_CLEANSE_H |